First of all: exuse my english, i hope you understand what i mean....
In front of the torpedo wall (is that the english name? the wall between the engine and compartment) it is a plastic-cover, also covering windscreen wipers.
Underneath, on the right side of the big box in center (might be left side if you have right hand steering) there is a magnetic valve with 2 "bulbs" sticking up (the magnets).
There is a known issue that this valve clogs with saw-offs from the engine, leaving you with only hot air, or in my case: hot air in the side and defrost vents, and cold air in center.
There is a new valve with the same part number, only ending with a B to fix the issue, however it MIGHT help with a good flush of the cooling system, and removing the 2 "bulbs" and clean it out...