Start with the control of the heater valve. It is a pulsed valve and it will have power AND ground when closed. The power will come when the key is turned. The ground is controlled by the AC control unit. In the "low" setting the valve should see a constant ground.

I usually confirm the situation by taking needle nosed vise grips with fuel hose over each "needle" and squeeze the inlet or return hose shut (the fuel hose protects the water hose from the jaws of the pliers) and then drive. By restricting the flow of hot water you duplicate the activity of a closed water valve. If that solves the problem then see if you have electric control. if you do then a bad valve is the logoical answer.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician