Break in a wire, two burned-out bulbs, wrong bulbs, breaks on the flexible circuits if you have that type on the backs of the housings.
With the ignition switch turned on, test for voltage on the neutral safety switch connector. One terminal must have 12 volts. If it's missing, look for a cut / corroded wire or a blown fuse.
All of this assumes you have the older three-terminal neutral safety switch that was used all the way back to the early '60s. On the newer designs, a smaller switch turns on a ridiculously-over-complicated computer circuit that turns on the backup lights. For those, suspect the computer. I'll have to dig up a wiring diagram if that becomes necessary to figure this out.
Saturday, July 16th, 2016 AT 2:41 PM