a 9volt battery should hold the memory on your car stereo for a reasonable amount of time.
I put a becker europa in our '69 911S since it was a factory radio car, and had a gaping hole in the dash, while doing the work, I got the aux in connector from becker and now I plug my ipod in while driving, it helps to give something to hum while at traffic lights or slow moving traffic....but mostly I enjoy the music of the car.
most beckers of the day have an aux in, for adding a cassette, 8 track or even a record player.
-Lewis