It sounds like you have the same problem as me. The air mix to the heater matrix is controled by a valve - so even if the heating/cooling system is nice and hot, the valve will be stuck on either hot or cold. It's apparantly common and easy to fix, located behind the bulkhead from the engine side (my previous attemps was to remove the radio and try it from the dashboard side.) I haven't tried mine yet, must do so as winter approaches...
See this link for more details:
http://www.usedcarmart.co.uk/forum/showthread.php?t=5345
Fuz
Fuzzy
November 2006
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