The way the early cars' back-up switch is wired, the back up lights do not work unless you have the park/headlights on. The power for the circuit is on fuse #3. I switched the wire on mine per the wiring on the later cars to fuse #12 to power the circuit with the key on and the gear in reverse.
The early wiring config is a safety hazard since your reverse lights do not go on unless you have your park/headlight switch on. We've redone three early cars here to the later config with power from Fuse 12.
Good Luck
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