I'm not sure if yours is vacuum or electric, but my 1997 2500HD had a problem similar to what you describe. Turns out there was a TSB to add a vacuum canister to the existing vacuum system... . problem affected my defroster door too. I know that I had to redo my vacuum line to the cruise actuator twice, because the line sat under my battery and acid had leaked on it and ate through. Hope these suggestions help.