How does the monovalve work?
 

The service manual says this about the monovalve: "The monovalve is a magnetic valve controlled by electronic switching unit for temperature control in a 5 second cycle. The valve is powered directly from the fuse box and grounded by the electronic switching unit for temperature control depending on deviation from actual and desired temperature. If ground is connected, the [valve] is completely closed."

What does that mean? Could the monovalve be controlled with a manually operated reostat? In other words, will the valve modulate from full closed at 12 volts to full open at zero volts?

The monovalve is cycled open-closed-open-closed by the ACC controller. It either gets enough power for the solenoid to close the valve or it doesn't and the valve stays open. Modulating the voltage won't give you precise control over intermediate valve positions.

If you want manual control, you'll have to use a pulse controller.

Sixto
91 300SE
87 300SDL
83 300SD

You would need a variable duty cycle unit with power output capable of delivering several amps.

Good luck,