Hello. I am looking for assistance in trying to track down the issues plaguing the heater system of my recently-acquired '91 190e 2.6. So far, I have changed the thermostat and did a coolant system flush and the engine holds good temperature, but the heat remains only slightly warm at best.
In addition, yesterday during my first drive on the highway with this car I realized that the floor vents were blowing completely cold outside air.
Anyone have any advice on what to check or what can be a likely culprit? Thank you!

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