Shouldn't be too hard to figure out. Put a test light on the wire and on the ground. If it is on when the ignition switch is put on, it is a battery charge line and it should go to your batteries. Should be a heavier wire also, #10 most likely. It has to carry far more amps than the back-up lights. If it only lights when the car is in reverse, Voila, you got it, back-ups.

Good luck.

Bill