Are the rear lamps regular dual-filament lamps like 1034 or 1157? Or are they LED lamps?

Originally Posted by Luray
but the back are both flashing at the same time in either direction. One side seams brighter than the other for the side that it should be on.

Do these lines describe the problem?
If signaling for a left turn, the left rear turn lamp is flashing bright and the right rear turn lamp is flashing dimmer?
If signaling for a right turn, the right rear turn lamp is flashing bright and the left rear turn lamp is flashing dimmer?

Sorry to ask so many questions, but it is better to know as much as we can before getting into real troubleshooting.

One more, 6 volt or 12?

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.