Well, you should use the remote antannae wire from the headunit, that is the normal amp turn on/off. If that doesnt work, you can splice the red line going into your headunit. That is the ignition line that turns on the headunit when the car is on. Be careful though, do not let the other end of the turn-on wire ground out when the car is on, it will probably blow a fuse.
If you need a completely seperate line, look at your fuse panel, and find a circuit that only turns on when the car is on (not dome lights and such). Worse comes to worse, run a direct fused line from the battery and put a switch inside your car, just remember to always turn it off, like you would your headlights.