Not quite sure what you're after, but if you're looking at an RCD300 (from a mk5), then IMHO, don't bother. They're sh1te. My mate's mk4 GT
iTurbo Diesel has a fantastic OEM unit. We're just putting a Parrot 3200 on it for additional audio inputs etc.Don't know if that's what you're trying to do, but also to put the RCD in there is an audio/aux input, but you've got to get it coded IIRC.
Hope this helps in SOME way
DAn

(AND you're local - I'm over in Leominster!)