I've dealt with quite a few heating problems so hear are the things I'd check if I were in your position.
1. radiator is completly full of coolant
2. see if thermostat is closing the whole way if not buy new one (worth replacing anyways cause for it and gasket costs less than $10)
3. check for corrosion inside radiator caused by people never changing the coolant (will look like white chalky stuff) if there is corrosion you may need to replace radiator and your heater core. But its worth running a cleaner through first which you can buy at any auto part store. (pain to replace heater core)
4. If none of those worked I'd bet my money on the linkage for the heat and a/c somehow malfunctioning and you'd need to tear into that and do some scoping.
