Hi, had exactly the same issue on my A6 this week! Garage wanted £450 to replace the fan\labour\regas. Got a fan from GPC for £60 and changed it myself one evening in about 1 hour.
The garage said they would need to strip the entire front end off, but after a few minutes searching the forums I found out that it could be done with everything in place. I did take some pics whilst I was doing it but haven't time to post them up. Basically you need to :-
1. Remove the engine covers and air intake (including the small part attached to the front grill with 2 screws) Disconnect the fan (on the A6 it is plugged in under a plastic cover by the ABS pump). I also disconnected the battery to be sure
2. The top part of the fan housing next to one of the 3 arms holding the motor in place has a small self tapping screw, remove this.
3. The whole fan then needs to be rotated clockwise about 20 cms. This will release it from the housing.
4. The fan can now be removed, it will come out, not easily as it will catch against lots of pipes, cables etc so you may have to remove a few things or tie them out of the way (The N75 is right in the way on the a6)
5. I then cut the wiring off my old fan in order to remove it.
6. Put in the new fan, again this is not easy and will require some more swearing and general cursing of Audi engineers! but eventually it will go back in.
7. I rerouted the cable along the top of the engine bay rather than its original route, and left the old cable in place, This makes the job a bit easier as I was short on time. I will revisit it at a later date when I have more time.
8. The aux\secondary fan should come on as soon as as the aircon is on so you should see it working straight away.
Good luck
