Flushing Heater Core
Symptom: One side of your car blows hot air, the other side blows cold. This would indicate that the heater core is clogged.
The best way to flush them out is to use a garden hose. Just disconnect either side of the heater core, and flush it out with water pressure. The heater core has two halves, make sure you clean out both sides. If it turns out to be clogged, you should clean the entire cooling system.
There are different things you can use to do that, I know a citrus flush is popular. I'm a big fan of just using water pressure. You can remove the thermostat and backflush the entire system with a garden hose with good results in most cases.
Refill the system with a 50/50 mix of zyrex yellow coolant.