I am trying to help a friend fix his tailights on a 1991 d250, he has head lights turn signals and brake lights but the park and tail lights are not working. Initially I thought there might be a problem with the light switch and at one point the park lights came on while I was messing around under there, however even after putting in a new switch the tail lights still wont work and now I cant get park lights either.

Any thoughts would help.

 

from the wiring diagram i am looking at there should be a black wire with a yellow stripe that runs from the headlight switch to the park lights and rear lighting. check fuse number 4.

 

all fuses checked and good. The problem started when he had trailer harness attached and pinched wires causing all wires to cross.

 

well if the problem started with the trailer wiring that's where I would start my search. black wire with a yellow stripe should be the one you are looking for

 

I had the same problem with my 81 and it was where the wire from the headlight switch goes through the fire wall.

 

When I read the issue before going through all the info you posted, I though "Ill bet it has a trailer connector and something is jacked up with that." Then I saw you mention trailer wiring. I would look there first, without a doubt.

 

I had a similar problem but only the brake lights didnnt work. Turns out if i pull my hazard switch it would prevent my brake lights from coming on.