96 town car-- wife stold me of light but noticable smoke coming from vents accompanied by electrical-burning "ozone" smell.
I investigated & smelled the distinct odor of overheated / burned / burning electrical component scent-- I saw no "smoke", though.
Cant seeem to find a source for the smell. My 1st thought was that the fan motor or wiring was doing something "bad" but, no odor under hood & the fan IS blowing. A/c's blowing cold, like it should.

The only time I had that problem was when the started was stuck and when I would let go of the key after starting, the starter stayed engaged. It drawed power from other places and cause this. But the smoke didn't come from the vents. Try this, is your A/C blower blowing air on HIGH? Might be the resistor board that sits in the A/C box. When mine went bad and I replaced the faulty components and it works fine now. How ever, it didn't have a burning smoke problem.... Also, check all your fuses under the dash and under the hood, also go through all the functions on the control head for defrost, heat, etc.. make sure everything works in blend cycle, (like 72 degrees) Then report back.