Okay ... I have to run the engine ... there is no other way to flush the heater coils ... The whole system has to be flushed and the water hose will be running along with the engine. I don't know where the thermostat is located ... or how hard it would be to replace it, how can I find out where it is? The coolant level, is pretty low, but the color is still red ... if fact it doesn't look bad at all ... I want to say if it ain't broke don't fix it, but it was been 17 years and I did buy the truck when it was a year old.