'99..sat on the passenger side for awhile tonight, cold outside..noticed that when the air intake selector is set on the "mix" position the passenger side blows cold...when set to inside or outside it works better...??? Driver's side is fine..
On most cars that selector is not a mixture control, but just a door that moves between 2 openings.
When you move to fresh air you want to close the inside air intake, this forces all the outside air to go through the heater/AC box.
Having both doors open allows untreated air into the car, just like opening a window.