Yeah the resevoir has a little sensor at the bottom of it that shouldn't leak. I thought you posted that you have a clutch fan... maybe I misunderstood is your fan on your radiator electric or on the crankshaft? You keep talking about a fan not coming on, but you seem to be talking about your heater fan, anyway, I am confused. So if you have an electric fan across your radiator backside, that should be kicking on when you get heated up. If you are on the crankshaft, it should run all the time. Those are the only fans that will ultimately matter.
Trouble shooting
I am guessing you fully drained your coolant, if not, you need to do that every year to get all the crap out (starting now).
Are you sure you have no leaks (like really really sure) because pressure leak is really the number one cause of overheating and you could be losing no coolant, just pressure, and be boiling to lose coolant. Pressure is how your cooling system really works. You could technically run straight water (it will eat your block) and not over heat because water boils at 100 C at 1 atm. At (sorry no more Si units) 15 PSI (ok I will convert, 1.02 ATM), you can go higher by like 10 C before you boil. (like people who live in denver and boil water at 95C because they are lower pressure). SO seriously you should not be capable of "overheating" (Boiling) with coolant unless you are not hitting high pressure.
Did you pressure check your radiator cap? Did you get a tight fitting pressure tester hooked up to your car and did they hit (what is it 13-15 PSI?) full pressure, and did they hold that PSI for atleast a couple minutes with no pumping. And, did no fluid leak from anywhere?
Last trouble shooting, is there fluid where there should not be (have you checked your oil for having water in it (light grayish sludge)?
I agree on checkin the carpet for being wet
Also, between your oil cooler (there are three coils on your front, outermost is the oil cooler, middle is the air conditioner, back most is the radiator) there is a small gap to the AC coils, and another small gap to the radiator. Check and see if they are full of junk like leaves and all.
And last thing why is your coolant green? It should be pink or orange colored (ok everyone tells me it is orange but I swear I think it is pink) You are using the Audi coolant right?
If you mean there are green swirls in it, Like kind of rainbow colored but mostly green and floating on the top of your coolant, that is not green coolant, that is motor oil. And if there is motor oil floating on top of your coolant, well then you have either overheated one too many times or found the source of your prob, (see the part about water where it shouldn't be) and um consider trading your car in for a new one.

 

wow thats an awsom reply, and this is a delayed thank you, ive checked everything youve said except the pressure test which i will do and all tests pass, the coolant is not floating green its all greenish, i flushed the system correctly yet when i added the G12 PLUS is seemed to start of pink then turn green im assuming from the old G12, i was told by audi that G12 plus is made so you can mix it with other coolants, even though i flushed the system correctly im assuming little remains of green mixed in with the coolant and turned it all green, its not oil as its all green from top to bottom, also i checked my oil and there is no coolant in there its pure oil and the carpet are fine, except when it rains and i leave my windows open :P

im just posting on other threads about my temperate raising and dropping above and below 90 degrees so often but when i turn my heater on itll stay stedy at 90 degrees hmmm?

people are sussing out temperature sensor, trying to get more info on it thanx for the help though

 

just to add to this , i noticed when i check coolant temp through my climate control itll read 80-95 degrees yet the pin would sit steady at 90 degrees? and itll keep going up and down from that range, so i dont know????