Hi, last week I went to turn my heat off on my 86 Cutlass Ciera and it seemed 'stuck'... then a small brown plastic cylinder cracked and fell on the floor through the middle of the dash.

Ever since this happened I get a hissing sound every time I turn the engine on, and the fan is blowing AC through the top vent when I'm driving down the road. I can still control the AC with the switch, the fan works fine and is adjustable, the heat will come on if I move it over, etc... but this hissing is getting annoying.

???

Thank you.

The heater/ac is controlled by vacuum. The hissing sound is a vacuum leak. I can't tell from your description what fell off but if you replace it or find the hose it was connected to and block it off with a golf tee the hissing should go away.