GMC 1992 Sierra

I purchased a 1992 GMC Sierra, and noticed that the climate control had a problem. When I try to go from defrost to vent to heat, it only stays on defrost. it seems that the contact in the button seems to be stuck. the button beside it which controls the fan speed,seems to work fine. Can it be that the contacts are dirty? What can I do?? Thanks.

Does the display fail to change, or does the direction of the air flow fail to change? If the display doesn't respond when you change it, its probably the control. If the display changes it might be the vent flap that redirects the air is stuck. Either way, its a chore to fix.