There is no damper in the ductwork. Heating is controlled by the monovalve. It opens and closes to allow coolant into the heater core. The heater core is ALWAYS exposed to the air stream, it's temperature is controlled by coolant flowing through it.

The definitive way to test the monovalve is to simply unplug it. It energizes to CLOSE, not to open. If you unplug the monovalve, you should have full heat 100% of the time.

If you don't, the monovalve cartridge is bad (you didn't say if they used genuine MB or aftermarket), or the heater core is clogged. It's really that simple. If they used a crappy aftermarket cartridge in the monovalve, shell out the $35 or so it takes to get an OEM one from the dealership or from this forum's sponsor and change it out. It's 4 screws and very easily DIY.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]