99 Jeep, 4.7L- Grand Cherokee Limited Edition (my other old Jeep, this is NOT the one that I just had the A/C repaired in

This 99 LE Jeep has separate heater (and A/C) controls for passenger and driver. A/C works fine on both sides. But when I turn on the heat on both sides, including the window defogger, it blows hot air on the passenger side but blows cold air on the driver side, at the same time, with both controls at the same setting (includes all the way to max heat on both sides). I cannot get any heat at all from the driver's side, but the blower is at normal speed (but with cold air instead of warm). Even if I only oprate the left side, I get no heat. Heat is always fine on the right side.

This vehicle only has one heater core, so how is this possible?

It's a rather serious problem because I cannot use the window defogger on the driver side, where it is needed the most.

I just hope to have it fixed before the cold weather comes. But I don't have a clue where to start.

-Don- Reno, NV