Headlights do not work,also seems like a short in the door locks when moving the headlight switch

Check the power wires between the headlights and the fuse box for damage. Sounds like you have a power line short to ground. Meaning the wire that supplies 12 volts to the headlight is making contact with a grounding wire or touching bare metal
somewhere between the fuse box and the headlight. Internet search a wire diagram to use as a map before you start the wire trace.

Check for wiring under the dash near the pedal. Make sure none of it is in the way of the moving parts around it.

this is caused by you delay switch or relay staying in the closed position causing it to short out . or if your headlight switch is on the steering colume and acts as a signal light switch used for low and high beam will need replacing

more than likely it is going to be a short in the headlight switch. not uncommom.

Clean the pin switches by the door jams, They are dirty or broken,

I had that problem before. Replace the light switch with a high quality NAPA or OEM Ford part.

It could very well be the dimmer switch. There is a plastic cover around the steering wheel held by a few screws. Remove the cover & replace the switch. First check headlight grounds for good connection. A common problem with well used cars is that after opening & closing the drivers door 2 million times the electical wires at the flex point where the door harness flexs tend to break. Open outer cover & pry back to expose the wires inside. The ones that are broken are the culprits. Hope this helps you & please recommend advice.

Your headlight motors are bad. Pre-87 cars had a terrible problem with this, upgrade them to the 87-up style. Firebird/Corvette motors are the same.

Replace the dimmer switch insid ethe dash it is usually separate from the headlight switch in these trucks. If not then get a known good switch out of a simial truck from a salvage yard before buying a new one and try that first. it could be that the wiring somehow got damaged. This is the only thing I can think of that may be of some help. the service engine light runs off the computer system where the other lights are illuminated from a separate circuit but if the dimmer control switch is moved and they work it is in that system...a short or a bad switch.

u prob have a bad ground on your headlights