What are the chances that a bad thermostat is why I don't have heat in the car?

 

Assuming it’s for a 4th Gen, it’s very common for the heater core to get plugged up. Can be flushed with a garden hose. Don’t get the Opti wet, assuming it’s an LT1.

4th Gen LT1 F-body Tech Articles

 

I’d recommend a new ACDelco thermostat part#131100, ACDelco radiator cap part #RC94 and very good, ginger flushing of the heater core. When I did my water pump last year I flushed my heater core both ways 4 times (the block and radiator too) and I can’t tell you how much crap, how much brown/black chunks came out in the first two flushings it was nasty.