Auto or manual?
I'm not sure about the auto's of that vintage, but the manuals should have a mechanical switch on the side of the gearbox for the reverse light.
The switch on the side of the gearbox switches +12V to the reverse lights.
One end of the reverse lights is constantly grounded, the other end connects to the switched +12V that comes from the reverse light switch.
And of course check the fuses and the bulbs.
Cheers
Wilbo
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