man the heater system is pretty simple. the only things i can think of is:
the climate control unit still does not work(used one)
the monovalve is bad( used one)
not enough coolant
or perhaps a vacuum leak
when you have it on heat, does air come out the center vents like the a/c would?

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual