FWIW my 03 CC has a dual blower single motor very similar in appearance to your 90 model that is located over the windshield with the two speed switch located on the dash. It seems to serve as a turbo type defroster for the large windshields in the XLII. The air supply is from the ceiling duct work that comes from the roof airs/heat pumps.

I have used it for defrosting the windsheild and also to cool the drivers/windshield area when sitting still with the roof airs on or while driving into the Summer sun at the beach, but no thermostat is hooked into it.

Everything is neatly concealed above the Drop Down TV and can be seen only wnen TV is down by a experienced eye. My first thoughts was it was the control box/motor for the overhead spotlight but when I opened it up it was a blower very much like what you pictured.