The most common cause of the 'stuck on hot' was the heater matrix bypass valve.
Beneath the scuttle panel you'll see the 2 water pipes going through the bulkhead into the cabin, just before they go inside there is a valve assembly with 4 water pipes and a small motor (2 pin plug).
when signaled the motor diverts the hot water away from the heater box and back to the engine. When heat is required the valve reopens to allow hot water to circulate around the matrix.
The system is controled by electronics built into the main heater panel i think so it's best to have it checked properly.
hay88, March 2010
