1985 W123 Defrost not Working

[wordmanjay]

1985 300D Turbo
The defrost was taking longer and longer to come on, then it just stopped coming on at all.
There's been a vacuum issue, which I have yet to address, that prevents the different ducts from opening. The defrost, however, has always worked until recently.
I've made sure there was enough antifreeze and I've checked my fuses.
What's next to check?

Thanks in advance.
Jay