Taillight Issues - 1968 C10

[Graham57a]

I am working through a list of issues on a factory 1968 c10 for a buddy and I am striking out trying to figure out a tail light issue that arose. When I got the truck the rear taillight harness was non existent so I pieced one together and used new tail light and reverse light housings along with new bulbs, lenses, etc. Once I put everything together it worked perfectly, about 3 weeks ago. Working through the rest of my list I noticed the other night that the tail lights were no longer burning when I turn the headlights on. I did replace the neutral safety switch and my reverse lights work and so do my brake lights. So that tells me that the issue is not a ground or a burned out bulb. My initial thought was a fuse but I have replaced all of the fuses in the fuse box and I specifically checked the tail light fuse with my multimeter and I have 12v on both sides of that fuse. I also thought maybe the headlight switch so I swapped it out for a known good one, still no change. My last guess was to check all of my grounds and double check the hot junction block near the battery on the inside of the pass side fender and everything checked out, still no change.

I did some searching through the older threads on here and I didnt see anyone with my specific problem so hopefully I am not the only one. Any help would be greatly appreciated.

Thanks guys!