Which wire?

I have a '96 Sonoma with a wiring issue? Or at least I think its the wiring! I have no tail lights, break lights but when I turn on one of the turn lights, they both come on steady untill I turn off the head lights then the turn lights will blink. Only when I turn off the head lights will the lisc plate light come on. I have checked the fuses and am working on checking the wiring itself. I have been looking at wiring schematics and don’t really know what I’m looking at or for. Any Advice would be helpful, Thanks, Lisa

If you have dual filament bulbs for the turn signal/brake lights, try replacing them in case the filaments are shorted together.
Check the ground connection on the rear bulb sockets. I put a voltmeter from the bulb socket ground to a metal part of the car. There should be no voltage reading with the lights turned on.

I have 96 s-10 that did this same thing and even had a faint buzz on occasion when pressing the brake. I noticed that shuting the hood affected this. Anyway, I couldn’t put my finger on it. About 2 months after this started 1 of the headlights failed. Swapped lights with no change. Replaced both… it stopped. It may not help, but you may start by checking the headlights or wireing near. Good luck!

No, they are not dual filament, but I replaced them anyway just to see if they both happened to have burned out at the same time. That is when I noticed the turn signal lights being rather ‘inconsistant’.

I will check the ground but before I start probing all over the place, I am wondering if there is a place where all these wires meet. Is it at the tail light bulb attachment? or someplace under the truck, in a bundle? does the wiring harness run down one side of the vehicle then over to the other side or does it run down the middle, then biforecate to the lights?
What has me baffeled, is the relationship of the problem. It seems odd that both tail lights & break lights are out, the turn signal lights come on steady (like tail light) until you turn the headlight off then the turn signal will blink. It’s like the lights are confused about what they are supposed to be doing?

That sounds like a classic ground failure.

Run a ground wire from a good clean ground near the lights in the back forward to the engine block or battery negative terminal.

The turn flasher, hazard flasher, and stop lamp relay go through the Turn-Hazard Switch. You should be able to get another Turn-Hazard Switch for a nominal cost from a salvage yard. I don’t know if that will fix the problem; but, it may.

did you get your ALT done?

The problem may be due to a bad ground. Check the chassis grounding from the battery. You could also make a ground lead and tie one end of it to the negative side of the battery. Then tie the other end to a good chassis ground and see it that makes a difference.

Looks like it uses a double-filament bulb to me.