This is a project truck that I bought knowing the headlights weren't working. I have changed the battery and alternator, the wiring harness doesn't look great but works for all functions except the lights. I have gone through the truck with a multimeter checking all plug ins and sockets. The tail lights were getting enough power but after changing the bulbs and letting the truck sit for a couple hours, the tail light sockets were no longer getting power. I checked them again with the multimeter to confirm this. The headlights (along with all connected front lights) are only getting around 3 volts, I've searched for a headlight relay but didn't find it. I am a little out of my element here as I am unexperienced in most regards to electrical issues.

Thank you in advance.

 

Take the dimmer switch loose from the floor. Turn the headlights on, and then probe the dimmer switch. You should have 12v coming in from the headlight switch on one wire, and you should have 12v leaving on one of the other wires depending if it's in lowbeam mode or highbeam mode.

MAKE SURE YOU LEAVE LIGHTING CIRCUITS CONNECTED WHEN TESTING WITH A METER. You will get a lot of false readings if you unplug bulbs and unplug connectors when checking voltage in lighting circuits. Always try to check it with things still plugged in.

 

That is good for the head lights but he also dose not have tail light
I dont know why the battery & ALT were changed unless they were bad. Battery not holding a charge with truck off and ALT not charging battery.

Lets start at the battery. Hope the cables are good, looking at them may not show they are bad.
More so the ground side of the system. Cable should go to the motor, motor to frame and a heavy gauge wire from motor to fire wall (cab).
If you want you can also run a ground, heavy gauge wire, from the frame to the bed for the tail lights.
You can NEVER have enough grounds!

With the battery disconnects I would pull all fuses and clean the ends of them and the holders as they get a funk and do not make good contact.
You can test the fuses before you put them in. IIRC 1 fuse is for tail light, head lights use a breaker built into the light switch.

I also would use a test light, non-LED type, when testing for things like this. It is either on or off we dont care what the voltage is. If the test light will light sowill the tail lights / head lights.

If the grounds and fuse work and checking the dimmer dont make the lights work I would replace the head light switch.
Dave ----

PS I believe when checking the fuses (installed) after cleaning the head lights need to be on for power to the fuse.