Troubleshoot trailer taillight wiring

When I noticed that directional signals and brake lights were not working on BOTH of my small utility trailers, I realized that it could be bad wiring on both of the old trailers, or something in the car. (or both).

Trailer lights DID work at one time on both trailers, but of course, both could easily have developed issues since then, I’ve been bad about checking them regularly. I only noticed the problem because I’ve recently removed the trailer lights on the smaller trailer for sandblasting and painting, and they didn’t work when I re-installed them.

That could easily be bad ground on the trailer, given new paint, though I did clean a surface where the previous wiring grounded at each tail light.

To discriminate between a problem in the trailers and problem in the car, I bought one of those small 4-flat testers at a local store. The tester indication matched what the trailers were doing: taillights work but no brake or directionals. So the problem appears to be in the car.

That’s a problem. I use the smaller of the trailers frequently. I had to look deeper.

The car lights are fine, everything works normally.

When I got this car a few years ago, it had no trailer hitch and no sign that there ever had been one. But inside the compartment for the jack was a wire harness with a square shaped connector.

At the time, I assumed that this wiring was factory installed, but looking at it now, I see that there’s electrical tape wrapped around it, casting some doubt on whether it was factory installed.

To get the car set up for towing my trailer, I bought a used trailer hitch and bolted that to the car, then bought a manufactured 4 wire harness extension to plug into the “built-in” harness. It did mate with the existing connector, suggesting that the wire harness in the jack compartment might be factory installed equipment. The reason for the black tape remains a mystery. Again, the trailer lights did work at one time.

Continuity test on the manufactured 4-flat extension shows all wires are good. So that suggests a problem in the car system. There might also be problems in the trailers, but the tester says there’s something wrong in the car wiring.

Can anyone suggest where to start in looking for a problem here? Am I right in assuming that I should have 12 volts in three of the four wires in the square connector on the “factory installed” harness when properly activated, and one functioning as ground? (I presume white is ground, is that correct?).

Again: all lights work normally on the car. The 4-flat tester shows good taillights, no brake lights or directionals, at the end of the wiring harness extension.

Thanks for any advice.

White is the ground. The starting point is checking all 4 outputs from the car. Missing 2, next is to unwrap that black tape to check how the ends of those wires attach to the car’s wiring. Wrapped bare wires are bad. So are wire nuts and Scotch Locks. Crimped or soldered connections are best.

There may dedicated fuses for the trailer harness. Start there. There also needs to be a converter somewhere in the line to allow the same filament in the trailer lights to be used as both brake and turn signal lights (assuming these are separate in the van). This might have gone bad or be missing entirely.

Thanks @Mustangman and @NYBo. I’ll dig in on this tomorrow. I have more mundane things on my plate for today. And maybe some other replies in the meanwhile will be useful.

Decades ago I learned the basics of 12 volt automotive wiring - completing the circuit just by connecting to ground; I understood that if a taillight was not working, the first thing to check was if it was not grounded due to rust or simply a broken connection.

But the brake / signal system has me stumped. I had hoped that the little 4 flat tester would say that all was well on the car end, and all I needed to do was track down a bad ground on the trailer. Having already bought a new 4 flat wire set to replace the dismal looking original wires, I assumed I’d eliminate the problem easily without even looking for it. But when I took the trailer out of the equation, implicating the car as the problem, I was caught off guard.

So now, besides looking for fuses and the “converter” I’m wondering where the wiring is hidden in my 22 year old minivan. If I have to trace that, it will be a big hassle because I have a platform built into the back of the Sienna where the seats used to be. Getting that out to remove any body panels to search for issues is an ordeal (but possible). I’d be more inclined to bypass wires than try to trace them…except that I don’t know where to start looking for where they come from in the front of the car.

I guess I have to be grateful that I’ve not been stopped for no brake lights since the last time I checked them!

Thanks for the replies.

I would start by checking for voltage at the plug on the car while using tun signals brakes etc.

Those wires very likely don’t run to the front. Whether factory or DIY wiring, only one set of wired for the van’s lights AND the trailer using the same wires. You need follow them only to the van’s tail light assemblies.

Thanks, that’s a BIG help, and a relief!

It should be fairly easy to track the wire back to the connection. The newer cars have fuses at the junction block but not so much for older cars. They just tie it in to the wires in the trunk for the various functions. I have one of each.

To check the car pigtail just use a $10 test light. Hook the clip on the ground (white), and then use the pick on each of the terminals to see if the light lights up. You have to have the lights on, and flashers going to check tail lights, brake/signal flashers. If you don’t have light on the test light on any of them, likely ground. Connect the test clip on a metal part and see if the test light goes on. If nothing anywhere, go back to the main junction block where everything is tied in. Don’t neglect normal wear points where the wire could be broken like where the trunk slams down on the wire.

I end up rewiring my trailer ever so often due to deterioration of the wires outside. I use my battery charger to in the same way to power the various circuits (lights, brake, signal). Using a separate test wire strung to the various lights tells whether I’ve got a problem with a particular wire or connection. When it gets too frustrating, I pull it all out and string new wires. The new LED lights are far more dependable than the old lights.

Four wires. White for ground, and brown, green, yellow. One for lights, one for right signal, and one for left signal. Which is which I don’t remember. You tube is your friend though or I think has lots of instructions for hitches and pigtail wiring and likely have a video showing how the pigtail is connected on your particular car.

Then weird things happen, seems like every few years I have light trouble of some sort. Last year after checking this and that turned out both bulbs were were shattered. Maybe they don’t make them like they used to, never unhooked the trailer lights before putting the boat in. I will from now on.

Thanks @Bing. Your reply was helpful. I’ve owned 12 volt test lights like what you mentioned for most of my life, so I understand the procedure you described. As for the 4 wire harness, the wires are labeled for right turn and left turn, so no problem with that. At the moment I’m deep into another and slightly more pressing project: excavating my workshop after a winter of neglect. Why have I been storing some of this stuff??

I’m pretty sure that the reply from @mustangman about the trailer wires going only as far as the rear light housings is key to solving this issue. I’ll investigate that tomorrow.

On my Acura the the box is plugged into a connector behind the trim above the rear wheel. On my other car they just junctioned it on wiring in the trunk. So yeah it’s all there in the back. Etrailer will likely show where. I used them for my Acura hitch and wiring.

Etrailer has a video showing the install on a similar van, the Curt harness plugs into the taillight harness on both sides, Just need to get into the area’s shown in the back of the van.
2000 Toyota Sienna Custom Fit Vehicle Wiring - CURT (

Thanks @wolyrobb. I really appreciate that you tracked down that video. I just didn’t have time today to devote to this repair, so hadn’t gone looking on Etrailer.

The video was extremely useful. Now I better understand the wiring that I’m working with.

I had a just little time tonight to poke around in that jack storage area. My home made platform with drawers makes accessing that storage area very difficult but not quite impossible.

I managed to pull out the harness which included a Hopkins brand converter box, with a square shaped connector. That was in the car when I bought it.

That is what I had connected the harness to (also Hopkins brand - from local Napa store) a few years ago. At that time, I didn’t go any farther than just plug in the new harness adaptor (square to 4 flat). So I assume that the Curt item is basically the same thing as what I already have (except mine has a problem somewhere).

Is the converter box an electronic device which can fail, or is it just a way to physically put five wires into four?

I’m hoping that there’s a wire that has been compromised and could be repaired without pulling the platform, but that’s probably unlikely. I don’t mind the cost of all new wiring, as much I’d like to avoid the effort of removing the platform! I’d be satisfied if I find a problem that can be mended.

I only had time tonight for a quick look. I used my test light on the square shaped connector to confirm what I had previously observed on the 4 flat wire: the contact for taillights is hot with lights on, but turn signals and brakes don’t light the tester when I activate signals or brakes.

I presume that the Curt product demonstrated is the same kind of thing that was already in my car when I bought it. Because I have the platform in my van which prevents me from removing those panels, I’m going to look for ways to pull off the taillight housing to see if I can find an obvious problem - there are two easily accessible bolts visible just by opening the tailgate, which might be what holds that entire housing in place. I’m not sure yet.

Maybe I’ll get lucky and find an explanation for my issue, and be able to fix it without pulling out the platform. But if I have to, I’ll get it out of there. It’s heavy enough that it’s a hassle pulling it out, but it can be done in about a half hour. I built this platform when I had a Grand Caravan, but the Sienna is smaller, so the platform barely fits in the Toyota (I had to trim it slightly on the sides at the back). There’s just enough space to allow me to get the jack out, but it’s really tight!

Thanks again for finding and sending that useful video.

Most likely a simlar product to what you already have.

Yes they can go bad

It is a set of diodes, which can fail. Given your symptoms, this is the likely culprit.

Is there a way to test and confirm that it’s bad before replacing it, or is this a slam dunk that the converter box IS the issue?

Check to see if you are getting the proper voltage on the input (5 wire) side. There will be separate wires for the brake and turn signal lights.

Thanks for that @NYBo. That makes good sense.

I made a quick attempt to do that last night but I couldn’t easily get to the conductor inside the wires on the 5-wire side of the converter box. By then, it was time to quit for the night! But at this point, I’ll try scraping away some insulation or just cutting wires to check for voltage.

If there’s voltage upstream of the converter box and none below, I’ll replace the harness. In that case, I’d consider just cutting off the converter box and splicing in the new one, even if I have to buy the entire harness, in order to avoid the hassle of pulling out the platform. But maybe the converter box is available separately to be spliced in? I just called the Napa store and they can get the harness but it will take a few days. Another store tells me I can buy just the converter box, and they have it in stock.

I’m grateful for the reply, thank you!

My harness from etrailer had the flat connector not square. There is also a fuse in that connector box. If half works, don’t know what that means. Might be worth the $50 to just start over with a new box. Might want to just give etrailer a call and ask them about your symptoms. They are pretty helpful.