Why install a switch or relay? I replaced the jack on the aux input with a radio shack 3-conductor 1/8 inch headphone jack. It has additional connections for pass through audio which I connected two RCA jacks that my DVD player plugs into. So, DVD player plays through AUX input UNLESS something is plugged into the 1/8 stereo jack, in which case, the ipod (or other device) takes the connection. Simple, and far better then kludging a switch or relay.
cjett