Rear heater is independent from the front. It has its own heater core and fan mounted under the passenger front seat.
If you look under the hood near the firewall on the left side you will see two black 5/16 steel lines coming up. These are the supply and return to the rear.
If its your front thats the problem id check that the valve that allows coolant to the front core is functioning ok.
Does it act the same no matter what postion the switch is in? (ie defrost,floor etc?) I had a vehicle once that had some of the ducting come loose and i only got heat in the defrost mode.
Plugged heater core could also be your problem if valve is working ok.
Hope this helps.
Unrelated to your front issue. If your in a winter climate zone inspect those rear heater lines under the vehicle,they rot out and leak in a big way. replace with stainless tubing.
