On almost all MB's the mono valve is just an electric heater value. Power turns the valve OFF & its spring loaded OPEN! Under most conditions the valve is Pulsed on for a second or two & then should pop back open to allow hot water into heater core. After a while the coolant erodes the rubber diaphram & hydro-locks the opening & closing operation. If it sticks SHUT you have no heat! ON you car its right by the battery & easy to replace. Before you start the car try unplugging it, small brown 2 pin connector & then seeing if the heater will work.