I never really noticed before, but in my work truck on the way home from a job I had my son drive (he needs the hours behind the wheel), and I discovered the passenger side vents only blow hot air. The driver side was blowing nice and cold. I never noticed before. I almost never have a passenger, and the passenger side vents are usually closed.
Where do I start?
P.S. My daughter exploded a Pepsi by leaving it on the dash a couple years ago. I suspect a lot of it ran down the passenger side windshield vent and may be contributing to the problem.