It's possible that the evaporator for the front is dirty and you aren't getting good air flow across it. But it's nothing even close to an easy job to fix.
It's also possible you might have a vacuum leak to the actuator that directs the air over the heater core. You might be mixing hot and cold air.
It's also possible you might have a vacuum leak to the actuator that directs the air over the heater core. You might be mixing hot and cold air.
