I wouldn't use just any old ignition sourced wire, some of those wires are emission control wires and tapping into one of those will trigger your check engine light. People make this mistake all the time usually when installing a stereo. Make sure you tap into a dedicated power source and not one for vehicle functions. Just because a wire has voltage doesn't mean it is usable for other purposes. If you are going to tap in to any old wire to trigger the relay you might as well tap into the lighter and I wouldn't suggest that either.