Greetings. My son has a 1990 Dodge D150 pickup truck (bought used from my uncle). A few weeks ago, it developed this strange problem. When the headlight switch was pulled out halfway, the parking lights would flicker then stay off. When the switch was all the way out, the headlights would come on but not the parking lights.
Replacing the switch with a brand-new switch didn’t change anything. All the fuses look good.
If the switch is pulled out about halfway thru the travel to the parking lights, sometimes the parking lights come on. But when pulled to the detent, they go out.
As far as I know, this is not a Canadian truck, so it doesn’t have the headlight module.
Only other thing to add is that he pulls a trailer, and I think this started when he started pulling the Class II trailer. But chasing this thru the service book doesn’t shed any more light on what’s causing the parking lights to not work on cue.
Help! Suggestions, comments welcome!
You might try changing one of the bulbs and checking again. You have to prove or disprove them.
It’s not just one bulb. When I can get them to light up, they all light up around the truck like normal. Dash lights work at that time, too. But they won’t stay on. All or nothing. And he has a job pending where he’ll be driving back home after dark, so I don’t want him to get in an accident.
I dont have Dodge wiring diagrams, but there should be a seperate fuse feeding the “park lamps” terminal of the headlight switch.
Looks like that fuse is blown or theres a break in that wire.
Not sure what “all the fuses look good” means.
Have you checked the park lamps fuse?
There’s 2 feeds into the switch, one via a fuisable link, one thru a 10 A fuse. I checked all the fuses, pulled each one and buzzed on a countinuity meter. That’s what I meant by “all the fuses look good”. 
Still trying to figure it out…
I hate not having a wiring diagram to look at, but I presume the 10 amp fuse is what powers the park lamps.
Have you checked for 12 volts from the fuse at the switch?
If it’s not there, check at the fuse itself.
If the above checks out to be OK, whats next up after the switch? Possibly a park lamp relay??
Have you got the wiring diagram for this?