If you mean heat from the center console for the rear, it uses engine coolant just like the main heat. Make sure the heater balve on the firewall moves when you move the lever on the dash from hot to cold. It could be stuck on cold position.
They both use coolant, nice thing about having a REAL aux heater and not just a duct from the main is if one fails in the dead of winter your not completly SOL. Your front one may have a clogged heater core.