I recently brought my car to an independent mechanic I know to have my AC fixed. I was expecting to need a compressor, and the associated $800ish charge. However, the issue turned out to be electrical. There is a hidden, common failure (I know, surprise surprise) fuse sub panel under the hood. And not the one you're thinking of up by the windshield with the relays.

You know the mess of wires and stuff that sits on top of your battery, under the battery cover? There is a small box in there, about 3 inches by 4 inches by half an inch. It has 3 traditional looking fuses (30 amp I believe) in it, and a bunch of monster fuses for high current items like the starter, and 5 or 6 wires running in. The 30 amp fuse, third from the left, turns the AC compressor and the radiator fan on and off (so if both of these are not working on your car, this is almost definitely your problem). That fuse was poorly wired, so rather than blowing, it melts and wrecks that electrical path. The entire box must be replaced. It costs about $130 from the dealership (a ridiculously cheap part for a TT, especially when your dealing with the AC), and the new one is an updated design that won't fail the same way.

I know some of you have the same AC problem I was having, so I hope this helps you out.

http://farm5.static.flickr.com/4115/4882027735_05779e2261_m.jpg

this will help sometimes it is just the left 30amp fuse that needs replaced but then if that blows it has a high chance that it takes out your AUX. fan Controller located under your battery two person job and the controller runs $304 from dealer but can get as low as $60 any who it will happen the odds are in favor of it

I just had the same issue and by happenstance came accross the fuse box and saw the left 30A fuse was blown and melted a bit.  Was able to jam a new 30 amp fuse in and the fan and air are working again....

This may not have to do with the A/C part, but IMO get a slimline fan or another good aftermarket fan. Your stock will break sooner or later as mine did!(130,000miles) not to mention it works better than stock and takes up half the room! as with any part that breaks on my TT I just replace with better.

How the heck do you get the fuses out from that thing above the battery??