If your getting the carPC install just for the added music storage, I personally say don't. Its not worth the hassle if you just want to add several gigs worth a storage space when you can easily add an ipod or something similar. But if you really want to go for the full carPC setup, bypass and get rid of the cd stereo all together and get a line booster similiar to THIS if your worried about line output voltage. Theres ones with higher voltage and different prices. Just search for line boosters on google, froogle, and ebay.
EDIT: I just noticed, your in the wrong forum. You should probaly post questions like this in either the newcomer or audio forums. Mods will probaly move this to the appropriate area.