Half Heater

I have a 99GTP and my heater blows out hot air on the driver side and cold air on the passenger side. I live in a state where its gonna get cold fast. i believe the problem has to do with a vacumn pump being stuck or not getting suction to open or close. I need to know how to get at the vacumn pumps behind my dash. My air conditioning worked great this summer. I bought the car in may of this year. Thanks for any and all help.