98 Grand Prix - only passenger side blows cold

There is a good chance that the vent door actuator has gone bad. It's common for them to go bad since there only made of plastic and after time the plastic get's brittle and the warm gears inside break. The dealer price is like $250.00 and Auto Zone is like $130.00 and you can get the step by step instruction to replacing it. Try this first, turn your radio off and everything else. Disconnect the battery for 1 min and then reconnect the battery, turn the ignition key's to the run position with out starting the engine and wait for the check/service light on the dash to turn off and then start the car and try your A/C with the different vent setting. This will reset the vent if it was just over ran and jammed. If there is no difference then your going to have to replace the actuator. You can always do the Salvage yard for $35.00 and they come with 90 day warranty, hint the 2007 Suburban, Silverado, Tahoe has the same parts so you know the part is only 2 years old. Good luck and hope this helps.