The repeaters we use are basically the same as in Mark's car - they sell under a few brand names, including Powerlink RCX (info here). There's also (unbelievably) a remote control extender website, and if you look under the links section, there's a schematic for a wired remote repeater, which you could probably chuck together for next to nothing. It's 5v powered, as opposed to the units we use, which are 12v and can be fed straight off the vehicle battery, but presumably the current drain's pretty small, so you could probably just hang it straight of a potential divider. The cable based system's probably a much better idea given the amount of interference floating around in most cars these days.

Dominic

Cambridge Car Audio
http://www.cambscaraudio.co.uk