I'm trying to get advise/help on fixing the heater in my 00' ML 55 AMG

Symptoms - Blows only cold air. (Very cold today as outside air temp is 20 degrees)

What i've found:
blower fan works
Recirculation pump had burnt up and fuse was blown. (i've temporarily bypassed the pump for now)
all other fuses are okay (k19 relay seems to be working)
Water pump not leaking
Coolant level okay
Engine gets to normal operating temp: 80 c
heater core doesn't leak (both in and out hoses are hot- indicating circulation?)

The obvious problem is the recirculation pump. But i am not sure that is the only problem. i would think that there would still be a little warmth from the heater because there seems to be some circulation through the core. In fact the heater worked about the day after i'd bypassed the pump. Hasn't worked since.

The a/c heater control (in dash) is one possibility... are there any other switches or controls to check?

Any advise is greatly appreciated.