2005 Buick Terraza with 60K miles and the problem we are having is this: When we turn on the Heater, only the drivers far left vent blows hot air? The middle vents of which there are two of them in the center dash, only the left middle blows warm (not hot) air, the passengers blows cold as does the middle vent on the dash closest to the passenger? Any thoughts on this? The A/C blows fine, just the heater is split oddly in this fashion. Thank you in advance.