Wild wild wild ... Back in 67, the reverse lamps weren't fused. If owner backed over a tricycle and and damaged a reverse lamp fixture just right, it would blow the main fusible link and shut down the entire car. So the 68 model harness has a white wire going from radios harness connector to bulkhead connector, only for a place for this wire to change colors again and be on a fused circuit. There isn't a wire in the engine bay at this port. So radio and reverse were on the same fuse. When I was helping a owner with his 72 model wiring, the wire added to radios harness connector changed colors at the bulkhead and went to signal flasher instead of reverse lamp switch. Best I can figure, they moved stuff around, did low dollar improvements until they had no choice but upgrade a harness with multiple fusible links and more fuses. Some of the vehicle could fail in accident but some of it would still work.
So... bottom line, tell us do you have aftermarket radio and that harness connector cut?