Need some help with a '96 buick park avenue with dual climate control. The heater blows warm air out of vents on the driver side, but cold air out on the passenger side. To make matters worse, the problem is intermittent. It seems, of course, that it only wants to act up on the coldest days, but thats not always the case either.

My reference material is a haynes manual that only covers basic heat/AC system. I have tried turning up the temp. control for the right side as high as possible, but makes no difference. I haven't a clue as to how to access blend doors and such behind the dash.