If it has a serial port, You can make a controller using a UART chip (see
beyondlogic.org) and 8 buttons (and maybe even lights!

). Each button gets a bit, and this is sent to the computer. I can (and have) whip up software to handle this that interfaces with Winamp v2. (I don't think Windows Media Player would be difficult).
If you want a LCD display to put in your dash, I might also be able to help you with programming, though I have no expierience in that.
Feel free to IM/PM/e-mail me with questions/comments.