I put some CDs in my car's stereo (6-disc, in-dash CD player) so I have plenty of music to listen to while detailing (except during the wash, when the windows are up )
Slacker Radio through the iphone. I'm a paid subscriber. For a few bucks a month I can actually download the stations I've created onto the phone via wifi so when I'm traveling I always have my music. Try doing that with other services.