I have a friend with an Audi TT, and I can tell you we're having a heck of a time finding an adapter to hook his Empeg up. Every shop in town we spoke with said "you have to rewire EVERYTHING". We know that is nonsense, but it's an example of how hard it can be to make it a plug & play operation.

If you're making a decision, I recommend deciding how much work you will put into rewiring, or how hard it is to finad an adapter, if you can at all.

Also note this is an Audi, and Bose systems in other cars (GM for example) may be a lot easier to plug in to.