The monovalve (heater water control valve) is located on the firewall next to the battery. It is plastic with two coolant hoses going to it. It has 4 small screws holding the wire coil and capule in from the top. Be carefull when removing the capsule from the wire coil as the connection to the two wire connection is fragile. BTW the part# for the capsule is 000 835 06 44.
Bosch Master, ASE Master, L1
33 years MB technician