My climate control - location and fix on 2002 Cadillac DeVille

when i turn my climate control on it makes a clicking sound for maybe like 30 seconds, goes away and then starts pushing air. It only blows air from the defrost and floor. every once in a while it will work properly but not often. Any thoughts to the problem, what part do I need and how do I access it?

Asked by for the 2002 Cadillac DeVille