Alright here's what I looked up
"Insufficient heater output may be caused by a faulty heater valve, by a misadjusted blending flap cable, or by a cooling system fault. Coolant leaking visibly into the passenger compartment is a sign of a faulty core or coolant hose. A sweet anti-freeze odor in the car's interior, or a constantly fogged windshield may also indicate a faulty core. Inspect the carpet and the area near the footwell vents for any moisture or coolant. A leaking heater core may be caused by over pressurization of the cooling system."
If that doesn't help I can look into it further.