I have exactly the same problem on a 1995 Bonnieville SE. All AC/heat controls function as expected, except when heat is turned on, it only blows hot air on the passenger side of the vent, defrost, or floor. At the same time, the driver side blows the same amount of air in the same places, only it is cold. Darn cold these days, even though I'm in the "south"
I'm not sure if the A/C is coming on and blowing on the driver side or it is outside air blowing in, but I suspect it is just the outside air. Turning on the A/C makes the air feel slightly cooler. Either way, It'* pouring out cold air on the driver and hot air on the passenger.
Any help will certainly be appreciated.