I would not rec. doing that. especially if it is a newer car. most cars run 98% by electronics and if you hook into the wrong wire you could cause major problems. a co-worker hooked his air fuel gauge up a few months ago, and out of the blue 3 days ago it stopped working and the tranny was all out of wack. I tracked it down to the upstream wire was at fault and sending false voltage to the pc, and tranny. so we disconnected it and problem solved. I would just grab another cd player and "do it by the books" a switch is an option but after a while you will forget to turn it off and drain the battery. if you dont wanna buy a new cd player take that one out and goto an electronic repair shop and spend 20-50bux and have it fixed.
just a piece of mind