Might as well trace them all.
The brake warning lamp wire (should be two of them, unless they changed for '77) would run up to the switch on the frame mounted combination valve. It's mounted to a bracket on the frame, near the fender/fuel pump area, and should be readily visible by following the lines from the master cylinder directly down to the combination (proportioning) valve.
The Black w/red wire is the one we're interested in now. Follow it up as far as you can and find out where it terminates outside of the main harness. If it's been cut off, you can still utilize it by splicing more wire to it, and finding a good 12v ACC source that's ON with the key.
Tie the power to one side of your switch on the shifter, and the Black w/red wire to the other. Voila! Backup lamps!
Paul