I've got a 96 GC with front and rear heating and A/C. All of a sudden with the A/C on and rear blower on, I get nothing but heat from the lower rear vent and nothing at all from the upper vents. Both heat controls are on full cold. Doesn't matter where source or recirculate controls are set. Works fine up front. Any thoughts?

Vent control in the AC/heater is actuated by engine vacuum. Locate and inspect for a vacuum line going from the engine to to the firewall for cracked ends or if it has fallen off.