There's a headunit around from Blaupunkt, I think its called Balupunkt Woodstock DAB53, and this plugs MMC cards directly into it.
I can't remember if theres one on the 'For Sale' section here or if I saw it somewhere else, but if you search for Blupunkt Woodstock on ebay, it should get you some results.
To be fair though, you'd probably be better off just getting a normal in car MP3 player headunit, as you can fit a lot more onto a CD than an average memory card.
Sony do a couple of really good ones, brand new for about £120ish I imagine (Mine was £150 about a year ago)
Steve