heater problem on 2000 Cadillac DeVille

heat comes out only on passenger side cold air on drivers side

It sounds as if you have a stuck air temperature door in your 2000 Cadillac Deville. Which ever side is not changing temperature would be the side with the fault. This can be confirmed by checking fault codes stored in the a/c control module using a scan tool. If it is confirmed that one of the temperature doors is not working, chances are it will need to be replaced. This type of fault has been listed as a common problem for the vehicle -