Heater Control Valve

Does anyone know what controls the water heater control solenoid on a 1998 Tahoe. I have the factory service manual and although it shows how to replace the soleniod and its valve there is no reference in the schematics as to what controls it. There is a reference in the schematic to the ground of the soleniod but not there control side. I have also found a reference to it as circuit 1596 on the C100 bulkhead connector but no additional information as to what it connects to. The reason for this is that the front heater is always warm, using the A/C it is never as cool and the rear and when on vent it is much warmer than the outside temperture. Yes I have checked both the mode and temeratue door motros and they and the door are both working properly. I have found no combination of heat or A/C control that will cause the bypass to be activated.

Tim