MustMakeAmends,

With the heater on max, you should get minimal airflow to the windshield - just a trickle. You should get air out the end vents and the footwell. If the defroster pod is bad, the defroster flaps won't close and you'll get weak airflow out the footwell. If the defroster is ok, then you may have a bad footwell pod that's only partially opening. (??) It's not hard to test or replace the pods, you really should pop out the glovebox and test all 7 vacuum lines. If the footwell or defroster pods are bad, they're easy to change. It's the other 4 that require dash removal.

Side note: Your 1986 model has two small round footwell pods... later cars (mid-1987 up) have a single square pod. If one is bad, you may get little to no airflow out either side. My 1986 300E had one bad pod, which caused similar problems to what you describe. I replaced it and the footwell vents work normally now with decent airflow.