First things first: what is your engine temperature after running for approx. 5 minutes? You should be at 80 deg C +/- 10 degrees. If you are down in the 60s or below, your thermostat is stuck open. Not only are you freezing but that's bad for your engine.

With a properly functioning mono valve, if you unplug it, you will have more heat than you can stand since the valve is failsafe to open with no voltage.

I suspect you have a blockage through the mono valve. Get a piece of 3/4" pipe and substitute it for the mono valve temporarily then go for a drive.

If you get roasted then the mono valve is the problem. If you are still cold you have a blockage in the heater circuit.

I would next try bypassing the aux water pump with the pipe and see what happens. If the impeller is jammed it might be killing your flow.

If that doesn't solve it (both the mono valve and aux pump bypassed) then you have a blockage in the heat core or hoses.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech