The heater core is likely clogged with sediment from the dex cool antifreeze. Disconnect the heater hoses, run water through the heater core with a garden hose in both directions. You will see the brown sediment flush out of the heater core. Flush the heater core until the water runs clear. It would be a good idea to flush the cooling system as well then refill the system with a 50/50 mix of water and new antifreeze.