ok so here is the breakdown on the heater units:
crappy manual heater knobs from 2003 i believe:
nothing nice or exciting about these... if you have it then i suggest you sell your car and get a new one that has a proper climate control unit as there is no way to convert it!
climate control units from 2003/2004 (not sure when they first came out)
available in single and double din versions, pretty standard units... side note - hold down the auto button for 3 seconds to match the passengers temp to the drivers side.
climate control from 2008:
will work on 57 plate cars onwards... double din climate units only... you can try to plug it into an older car but all the functions just wont work properly. look nice and updated.
modified climate control units:
this is the hybrid that i completed which has the chrome knobs on the older climate unit so that us who have older cars can still have the bling :)
details for the conversion is here http://www.audi-sport.net/vb/a3-s3-s...-switches.html