This is a common problem with these cars. It is an easy fix by climbing up onto the roof, and remove the top cover to the fresh air vent. Then there are about 10 or 12 Phillips head screws holding the next part on. The screws go through the roof, and into a 1" thick wood ring under the roof panel. The headliner in the patient compartment is stapled to this wood ring. You will also need to remove the chrome air diffuser from inside the patient compartment, and disconnect the power lead. Once you have everything removed, clean up any rust under the housing, and paint with a spray can of self etching primer. Assuming that the wood ring isn't rotted, all that you have to do is to reinstall the fan motor housing to the roof, with a ribbon of 3 M body coulk between the housing and the roof panel.
If the wood ring is in poor condition, then pull the staples out of it that hold the headliner from the bottom and remove the ring. Make a pattern out of the old one, and cut a new one from pine. If you have difficulty reinstalling the wooden ring, you can cut it in half, and insert each side separately. It is only to hold there to screw the fan motor housing into, and to hold the headliner up. It isn't seen once the air diffuser is reinstalled. Once you take the top cover off, these instructions will be much clearer to you.
If the wood ring is in poor condition, then pull the staples out of it that hold the headliner from the bottom and remove the ring. Make a pattern out of the old one, and cut a new one from pine. If you have difficulty reinstalling the wooden ring, you can cut it in half, and insert each side separately. It is only to hold there to screw the fan motor housing into, and to hold the headliner up. It isn't seen once the air diffuser is reinstalled. Once you take the top cover off, these instructions will be much clearer to you.