When you apply 12V to the monovalve, it will stop the flow of coolant to the heater core. Actually, it gets 12V anytime the key is on, the ACC system provides the ground as necessary. In defrost mode, it gets a solid ground signal from the PBU; in other modes with the temp wheel on min, it gets a solid open(?) signal from the PBU. In other modes, the PBU passes through a signal from the temp controller. Maybe you can isolate the problem with this info.

Sixto
95 S420
87 300SDL