Ac unit smoking, please help

[cfan10]

OK, I turned the air on a few days ago, just to see if it would work and it obviously needed freon. So fast forward a few days, needed to use the defrost and I smelled something like melting plastic, saw smoke from around the ac unit in dash. I then disconnected the whole unit and in trying to do so burned my fingers pretty good. Does anyone know the cause of this?
As always many thanks for your time. It is greatly appreciated.

90 suburban 350tbi

[Cabnchassis]

Mine had a ton of corrosion on all the connections behind the dash. I'd turn on the heater and nothing would happen, hit a bump and it would make a sparky noise and turn on. I'd guess mine was like that from either the windshield leaking (it's a gasket type not glued) or the cab lights leaking (it's a dually).

[cfan10]

Took it apart nothing is rusty or appears to have been wet. It is scary as hell thought it would catch a fire.

[Titomars]

It is most likely your blower switch or the terminals going to it or both. 30+ years of high current going through them builds up resistance and eventually..........poof !!