1992 Pontiac Grand Prix On 3 1 Heater Control M Hoping Get On

I have a question on my 1992 Pontiac Grand Prix 3.1 heater control I'm hoping to get some help on. The buttons across the top active the heater fan speeds just fine. However, it doesn't matter if I go to heat, vent, or defrost, the outcome is the same where the hot air blows through both, the defrost and the heater vents below (switching buttons back and forth do nada). The OFF button and fan speeds work but otherwise, it's warm air through just the defrost and bottom and I'd really like to get 'vent' working if possible. Anyone familiar with what might cause issues like this? TIA