Backup Switch Wiring

They aren't all the same. Far from it actually. So did your 68 have the reverse lamp switch mounted on the floor shifter or on the transmission? If it was on the floor mounted shifter, there is another connector at lower end of steering column where a column mounted reverse lamp which could have been. In this case the console harness is a add on extension.
Your radio has 2 connectors. One of those has a red tuner power wire and a orange dial illumination wire. How the factory harness for a manual trans build tied into this red wire? I don't know. In some cases they used a special little tap harness with connectors to go in between 2 standard connectors. The trunk lamp harness is another example. ( This would explain the short length of white wire described above ). Anyway...
Short of buying the alternate under dash harness, You'll need to cut and splice or use a wire tap to get 12 volts from the red radio power wire.
If it were me... I would follow the console harness back into the cars harness. There is or was a hot purple wire there somewhere. Your hot purple wire may have originated in the fuse box. If you don't utilize it, it's just floating about somewhere, so you still don't have exactly like ma mopar did it.