overheating wrx

I have a 1993 subaru wrx, has had, water pump, gasket (not Head gasket), thermostat, radiator, and heater unit where the radiator cap is, replaced with new and some good used parts, but the car still over heats, i have noticed that the heater dosent work when on hot, and full blast, someone said it is more then likly the heater is blocked, but i dont know how to fix it and is there any way of doing a home job on fixing the problem, as i dont want to pay a mechanic to do this, when i could do it my self!, the head gasket is fine, as there is no gunk in the oil, there is still full power when at full throtle but the engin still over heats, i hope some one can lead me in the right direction to helping me out thanks everyone!