Originally Posted by imidazol
I checked my service manual for 98s and it shows that Bonneville has a passenger temperature control and a driver temperature control. It'* possible one is not working. But my 98 (leSabre) has always felt a little colder on the driver'* side then the passenger side when it'* in cooling mode and the car has cooled off.
It'* possible you have a low charge making one side of the evaporator colder than the other side (hasn't someone mentioned that - maybe on 2000+ model) or you might have partial blockage off the evaporator core from crude that gets inhaled into the intake. Someone has explained how to open up the case in the blower area and flush the particles blocking airflow through the evaporator.
No separate driver/passenger temperature controls for the Bonneville until 2000. The FSM'* are for all H-bodies and show the LeSabre and Old'* eighty-eight as well. All the different cars systems are combined/jumbled up in the sections. If it does show sperate controls for the Bonneville climate controls, then it is a mistake.
The rest of the diagnosis seems possible. If any restriction is blocking the air flow, it may be deflecting the cool air to one side or the other in the distribution channels.