Ant,
thing is with these cars I have to make things like that. I have found if you upset one of these cars it will stay mad at you for a LONG time. the later cars ('04.5+ ) are straight CAN and really do not like being fiddled with.
background is electronics eng. mixed with autoelectrics.
find someone who has a electronics background they can knock that up in no time. (I'm in the midst of doing one now for a car.)