Bringing this back up as the issue continues.
I swapped the push-button control as I was recommended to do, yet this did not solve the issue. I checked the aux water pump and it seems to be working (you can feel it if you put your hand on it). Additionally, I tried disconnecting the vacuum hose from the heater valve, yet this did not give any better heat output.
So can it be a clogged heater core? Interestingly though, the return hose from the heater core is hot so can that rule that possibility out?
Can it be the water pump somehow not creating enough flow?(However the car doesn't overheat)

The car only blows slightly warm air when at idle and moving below 45mph, but above those speeds the heater gives up.
Any help would be appreciated. Thanks

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1991 Mercedes-Benz 190e 2.6- FOR SALE
1998 Audi A4 1.8T