72fastback

OK so I am installing a mega shifter in my 72.The back up light switch is on the transmission it has 4 wires 2 red 1 black and 1 white.I figure one red for the black and one red for the white {right I hope}. Now the engine is out of the car so I plugged the main wire to the battery so I can have the electrical system to work and it does all lights radio everything comes on but when i put the shifter in reverse it triggers the switch but the back ups wont light up.If I am doing something wrong please tell me and if you have done this before help would be appreciated .

thanks a lot rookie in training