back-up lights - '68 4spd barracuda

I would start at the switch on the transmission. Pull off the connector, turn the key to "run" and see if you have power at one of the posts. With the gearbox in reverse, check with an ohmeter to see if the switch is closed.

The wiring path from the switch to the lamps is

up the firewall, through the bulkhead connector, down to the left kick panel connector and back to the rear sockets. So there is two connectors could be no connection. don't discount bad bulbs