http://www.mkivsupra.net/vbb/showthread.php?49871-Heater-Maximum-Output-Temperature-Too-Low&highlight=heater+controls
Post 16 page 2
Hopefully the following may be of some use and potentially save someone a good few hours of wasted time...
It did turn out to be the air mixing flap that was causing the problem. When looking from behind the left dash panel (as mentioned above), I could see the actuating arm move ok when the heater temperature knob was turned; what I didn't realise at the time, was that a rod connected to the actuating arm goes down through the heater unit and ends up at point 'A' in the first photo (which is looking up towards the heater unit from the passenger footwell).
The actuating arm had come off the air flap lever end as at point 'B' in the second photo (just taken it off again for the photo )
It should be connected as in the third photo at point 'C'.
After spending quite some time flushing the system out (which was clean anyway), the fix turned out to be very simple and took no more than a minute or so. Anyway, I've now got a heater that when on maximum it could strip paint