My housemate has a iPod Dock mounted on his dashboard.

The dock cable goes under the dash town to the cigarette lighter (inside the ashtray, so you don't see it sticking out), and then the line out goes into the Aux input on his reciever.

It's pretty slick, he puts his iPod in the dock and it powers/charges it and plays it through his reciever. However, doing this involves ripping your dash apart, but so long as you don't mind doing that, the actual technical stuff isn't, well... all that technical.