I have a 2003 Buick Rendezvous. The drivers side blows hot air and the passenger side blows cold air. The car has that dual zone climate control. I have no clue where to start.

The actuator that 'blends' cold with hot is stuck in the cold position. There are a couple of options. Disconnect the battery, then drive the vehicle around without using the AC for 10 min, if the AC is defaulted to come on after a battery interuption, then shut the car down then turn it on again. You are trying to get the electronic actuator to reset and resume proper function. Another option is to try this: Start the car, adjust the pax side full hot, drivers full cold, wait 30 sec then shut car down, then start car back up and reverse the settings, pax full cold, drivers full hot. In both solutions, the vent actuators are stuck at either the full open or full closed, so you are trying to get it to 'self check' and allow them to resume proper function. After two years, I simply removed them altogether since I live where it is hot an only need cold.works just fine....