A B7 A/C unit will not work the Rear demist circuit in a B6 and does require a wiring loom modification with a 30A relay to get it to work. I have previously posted this but it was not so great to search for it, so have updated it with extra relay terminal info, which has recently been requested.
The relay terminal numbers indicated below are for a standard 12v 30A Automotive relay, typicsl relays shown at this site Relay Schematic Here, you can use a 4-pin or 5-pin relay as terminal 87A is not used here.
Relay coil is across terminals 85 & 87,
Relay switch Common is terminal 30,
Relay switch Normally Closed is 87A (No coil voltage and relay switch is connected normally between 30 & 87A),
Relay switch Normally Open is 87 (apply 12v to coil to make relay switch contacts between 30 & 87)
Any B6/B7 A/C unit requires coding to the car VIN - talks to the Instrument cluster for security - I guess to stop people nicking and swapping these easily removed items as they will just pull out!!
A/C removal - Single DIN is a doddle - just pull it out. Double DIN - I had to pull out & remove the radio and then I could pull out the A/C unit - Auto transmission lever needs to be pulled back twoards the handbrake first. Both instances no screws - they should pull out enough to remove the cables.
VIN coding of the A/C unit can be done through VAG-COM - do have the procedure somewhere if required.
A B7 A/C unit will not work the Rear demist circuit and does require a wiring loom modification with a 30A relay to get it to work. Thought I'd posted the modification but can't find it but have the text which is pasted below..............
.............With the RNS-E out this gave me access to the "newer" A/C unit I had installed when going from Single to Double DIN.
After mod. Rear Demist now comes on and the rear glass definitely gets warm - although I do notice that the relay pulses on and off and then on, say every 2-3 seconds. No errors with VAG-COM. Maybe it's the outside temperature being 20+ degrees? VAG-COM does register the relay switching on and off through monitoring the A/C unit, "channel 012", I seem to recall.
Can't seem to get the heated wing mirrors on - but think they do not come on unless outside temp is 5 degrees or less. Today is 20+deg. They are actually controlled via the individual door modules - probably over CAN-BUS and not with this heated rear window circuit.
Required for A/C units where Rear Demist is controlled by slave relay and not directly by internal switching of A/C unit. Bentley circuits suggest A4 models "from November 2004". I can confirm this mod is required for A/C unit part no. 8E0 820 043 BM.
The mod. was pretty straight forward and required a four "blue" female spade crimps, a short length of 1.0mm wire and a 12v/30A automotive relay.
Pull fuse F26 - Heated Rear Window - for safety!!
Pull all plugs from A/C unit.
Sourced a ground of A/C connector D13 - removed the hood casing - Soldered a wire here inside the plug and then remounted the casing. Terminated the free end in a female spade connector and this wire goes to the relay coil, terminal 86.
Undo the taped loom wires to connector "F" - you need to pull back the tape around 2~3inches.
Cut 4mm wire F6 (White/Lilac) around 1.5 inches back from "F" connector.
Cut 4mm wire F5 (White/Yellow) around 1.5 inches back from "F" connector.
- A/C end - soldered a "thinner" wire to this and terminate in a female spade connector which then goes to the relay coil, terminal 85. This is the control signal.
- Car end - put a female spade connector on which goes to one side of the relay switch terminal 87. This is the feed to the heated window element.
Re-taped all looms with PVC insulation tape.
- A/C end - insulate as no longer used to supply 12V/30A to A/C unit.
- Car end - (From fuse 226, i.e. Large 30A Fuse in position #26). Put a female spade connector on and wire goes to other side of relay switch, terminal 30.
Refit all A/C unit plugs.
Refit Fuse F26.
I also cleared any DTC errors with VAG-COM - I tested at various stages to see if relay switched and the rear demist light would stay on before connecting the 12V/30A power feeds! Just to be 100% sure it was going to work!!
Hope this helps.