had a similar problem on my 61. after spending two weeks testing all of the wires and fixing some really poorly done wiring repairs i discovered the real problem. i ended up doing two things to fix this problem. the first thing that i did was switch the grounding clips that the mounting screws hold the tailight assemblies on the truck with. i also took a piece of sandpaper and sanded all of the parts of the tailight assembly that need to make contact, i sanded them until they were good and shiney. my tailights are brighter and now they also work under all signal settings. makes me want to do the same thing for my really dull headlight beams, just to se if it works.

also i would check the turn signal flasher that is mounted on the fuse block. another thing that i did but forgot until now was change the little relay box on the steering column, not the signal itself but the little relay that turns the mechanical signal input into an actual electrical signal.

I ended up doing a lot of work and changing but what really fixed things was the new grounding clips and the cleaning of all contact surfaces in the tailight assembly.

good luck friend and may the electrical gods smile on you while you zap and twist your way to a happy truck.

"Never tell people how to do things. Tell them what to do and they will surprise you with their ingenuity." - George S. Patton

My Machine