if you have an aftermarket head unit, there should be a blue wire (usualy with a white stripe) that is on the pigtail in the back of the unit, might even be labeled "REM"
if you still got the stock head unit, find a switched power source to tap into
you should have 2 power wires to your radio, one is constant for memory and one is switched, IIRC the yellow is a constant and the switched is red, but anyways tap into the switched wire