Parking lights on a 91 chevy 1/2 ton pick up truck

https://www.fixya.com/cars/t25755726-headlight_relay_91_chevy_caprice

and it is a known issue. No fuses, hidden relay.

https://www.impalassforum.com/threads/headlights-dont-work.247307/

https://www.google.com/search?q=1991+chevy+caprice+headlight+not+working

..

I assume no lights work at all ... not parking lights or low or high beams...
Therefore the most likely reason is no voltage to the headlights because of a bad headlight relay, fuse, module, headlight switch, dimmer switch or wiring fault.
The relay is often located in the power center in the engine compartment, but there may also be a fuse in the fuse panel under the dash. If you do not have new relays/fuses in the vehicle you are sort of stuck.

check headlight fuse

I probably should not be answering this as I'm no expert, but ...
Let's see now, DRL comes on when in gear and headlights off, emergency break off.
Silly question but, are headlights working manually (click on manually and verify both working)? High beams, low beams OK?
Check the fuse. The DRL fuse in the underhood fuse box and the label tells you which one. I think it's 31 or 32 but not sure.
Aftermarket lamps (esp the stupid blue lamps) can cause this if a higher wattage is used. (Burns the plastic lamp sockets).
Also check the DRL relay.

First you have to make sure the bulbs are good, if they are check out good, you need to check the fuses and switch.
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New Slim hid ballast

did you replace the dimmer switch on the dash by the headlight switch

most likely have a bad headlight switch. It is possible for parts of the switch to burn out while the headlights continue to function. headlight switches are cheap at your local part store and pretty simple to install

change the headlight switch!! itself its broken inside the unner contacts

The headlights have 2 fuses. One is a main power feed under the hood in the distrabution box and the other is under the dashboard. Check the fuses and then check for power at the switch. If you have power at the switch then its not a fuse its the switch and the trailer you had connected most likely shorted and back fed into the parking and dash light circuit. Those circuits also run from the light switch.

possibly the dimmer switch on the floor? happened on my 91 half ton dims didnt work but brights did and then they cut out altogether. a little water goes a long way