Typically outside or recirculated air is a separate function from heater core or evaporator. The default for a bad pod is to flow air over the heater core. Getting outside air to the footwell without going through the heater core seems impossible based on the architechture of other MBs. Maybe it's so cold where you are that the heater core can't warm outside air enough.

Some places now sell replacement diaphragms for these pods. The replacement diaphragms work passably on single function pods; not so well on double function pods. Remember that those pods are over 20 years old and very brittle so use light even pressure if you decide to take one apart.

Anyway, I hope you get heat soon.

Sixto
93 300SD