brake pedal switch?

That will be hard to do on a modified harness. Do you still have the original signal switch connector at the column, or has it been cut and redone?

You can download a factory manual here

http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617

Or here. There are also simplified diagrams in the wiring section, sometimes incorrect and sometimes leaving out options like AC, but sometimes, simpler to follow

http://www.mymopar.com/index.php?pid=31

From the 65 manual, page 8-96

This is the original column. So the wiring colors up into the column should still be the same. Wire D4A to pin 7 is the stop lamp, that comes FROM the switched side of the brake light switch on the pedal. Look at the chart. D4A is a no18 RED wire.

I cannot help you with a power source. Nobody except the guy looking at the car can possibly know how that harness was installed.