I'm not sure if this might help, but here goes.
- The auxiliary blower motor is inoperative
- Fuse.
- Circuitry short/open.
- Auxiliary blower motor resistor.
- Auxiliary blower motor.
B/c you have the rear air option, you have a 2nd set of controls and parts to blow heat and air out the back. Take a look at your circuitry, fuses or any shorts. It also could be your auxiliary blower motor going out or its resistor.
hope this helps.