Rear Heat/AC Problem

I have a 2001 Ford Expedition - Eddie Bauer Edition, and I am having a problem with my rear heat/ac. The cooling works perfectly fine, but if I were to turn on the heat, it continues to blow cool in the rear. I've checked the rear heater/blower and it seems to be working fine. Is there anything else that would be causing this problem?

Does the heat in the front work?

Yep, it works perfectly fine. The rear cool air works perfectly fine, its just the rear heat that doesn't seem to work.