Don't let the neighbors see you do this, but this is what I did on mine when the system was gunked up. Drain coolant, disconnect the hoses from heater core, use garden hose at full pressure with a trigger nozzle to blast water one way through the heater core, then the other. Did that repeatedly. I mean repeatedly. There was rusty, muddy, nasty stuff flowing out both ways for quite some time before I was getting nice clean water. And I would pulse the trigger, like full blast on, off, on quickly. After doing that, the heater finally worked! And the coolant stayed looking nice until I ended up selling it. The previous owner had replaced the transmission and once I fixed a few little things, I really loved that car. Drove very nice and got respectable gas mileage for its size and being a 98 and all.