1973 DT3.  I have a new main harness and I am rewiring with a vape ignition.  My rear brake stop switch has three wires, a red, a yellow, and green with yellow stripe ( I know the front brake stop switch has green with yellow stripe). When I look at images of the rear brake stop switch by part number, it has two wires, a yellow and a brown. I think the yellow would go to the rear stop light, and the brown would be DC power. Don't know how to wire this 3 wire stop switch.

Either get the correct switch or test the one you have and find two wires that connect together when switch is activated and disconnect when brake released.  Tape off the third wire.  

I checked my new main harness again and it does have a light brown wire and green wire coming out of the harness for the stop switch.