Air conditioning blower problem on 1998 Cadillac DeVille

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
The air sometimes comes thru the vents, and other times thru the defroster. I've been told I need something called a "high-low switch" ... any help?
(1) Answer
The air distribution on your Deville is vacuum controlled. Before replacing any parts make sure all of the vacuum hoses under the hood are properly connected with no vacuum leaks. Also make sure the vacuum reservoir and check valve are not leaking. If everything under the hood is ok the programmer controls the vacuum inside the car, that could be at fault also. As you can see, there are a lot if things to check before replace the "up-down valve", as it is correctly called.