Even with the ignition switch off, you can change cartridges in the changer so there are always some hot wires in the cable. As soon as you hooked up the cables, the changer would start checking to see what's in the cartridge.

Actually, I forgot to disconnect the battery when I did mine and I was lucky I didn't blow a fuse or do some damage.

As with any electrical systems, it's best to turn off the power when working on them.

After installing this changer, I'd never buy another car w/o a CD changer, preferably in the trunk. Any changer I'd get would have to take a cartridge. I don't want to carry around a bunch of loose CD's.