[fredjacksonsan]

Check for clog....unhook the two heater hoses, and get your garden hose out. Hook it up to the inlet on the heater core and turn the water on. You should be able to see how much is flowing out; compare vs how much comes out of the hose. It may be completely clogged, or you may be able to use one of the flush kits for it; no guarantee, just might need to replace it.

Heater hoses easy to spot, they run on the passenger side of the engine, usually together, and terminate at the firewall.

If the coolant's new, then yeah just save it.

Thermostat is an easy replace job. The heater core is a tough one usually...