1976 dodge 1 ton 300 series wiring problem

[austinpalibroda]

i'm trying to solve some wiring problems on our 76 dodge truck at work. the clearance, interior and dash lights do not work, but everything else does. when messing with the clearance lights, there is power at the fuse, and power just past the fuse but no lights come on. when hooking power up direct just past the fuse, the lights will come on... anyone have an idea of why this would be or what to look for? we had a mechanic look at it and he cannot figure it out. any help would be great! if this is not enough info, please let me know and i can try and describe it better! thanks!