Something like this?
http://www.xcarlink.co.uk/product.php?productid=64&cat=16&page=1 Or, I have seen normal set to boxes used with IR extenders (or jusr resolder the IR sensor on longer wires and position in the car yourself). You just need to go to maplin and either get a power supply to run it from 12v or a 12-240v converter (they don't pull too much current) The biggest problem is the reception, but lots can be done with a signal booster. Remember something like the kit above is, at the end of the day, a re-packaged version of a set to box anyhow. If you have some AV skills you will be fine sorting a DIY solution.
Have a poke about here,
http://www.mp3car.com some of the guys there have done some cool stuff.