In an unpowered state the monovalve allows coolant to flow to the heater core. I don't know how this translates to movements inside the monovalve mechanism.

Rather than override the function of the monovalve, put another valve upstream of the monovalve. I don't know what car with ACC has a valve that's closed by default but I imagine MB being what it is is one of the rare manufacturers that uses a default open monovalve.

You might think of fixing whatever's wrong with your ACC so the monovalve isn't open when it shouldn't be.

Sixto
95 S420
87 300SDL