Heater blowing cold air on my 2001 Pontiac grand am Gt V6

I would suggest that something must have been disrupted having the stereo installed. It seems too coincidental. The temperature blend door motor and wiring, which switch the doors in the plenum from hot to cold for air flow, reside behind the dash, perhaps something got disturbed. It's worth having the folks who installed the stereo recheck the area of their work.