I wish I could plug my laptop's audio output into my car's stereo system ... my car's stereo rules (compared to a laptop).
Unfortunately I don't have a tape deck, so one of those cassette adapters won't work, and my CD player and amp are one unit (stock), so I'd have to buy a new CD player and new amp before it'd even be possible to plug in other sources.
Unless ....... maybe my deck has support for a CD changer? I think it might. But that's probably some proprietary connector.
Anybody in the Seattle area do custom car audio installation work? :-)