I just picked up a 1993 F350 Dually.The tow lights and indicators recepter does not read power all to but one post.Until I can really get into the wiring harness underneath,is it possible to have a relay go bad?The fuses for the towing package seem to be fine.I have no running lights as well as brake lights.Just the right turn signal works on the recepter.FYI,it's a 7 blade.Thanks

 

Do some searching under the hood for little mysterious boxes with covers. You may have one of these with towing relays, and some dedicated fuses just for towing if you did not find any in the main fuse box. Look over on the driver's side inner fender near the brake booster.