Grand Prix problems

I recently purchased a 1999 Grand Prix 2 Door. The car is equipped with dual passenger control. When switching from hot to cold the passenger side switches almost immediatly but the driver side slowly switches.

For example, If the temp is set at 85 and i set it down to 65 for the whole car just the passenger side switches right away. The driver side takes at least 30 minutes but then is as cold as the passenger side.

If anyone can help me with this problem it would be greatly appreciated.

Thank You very much