With all that said, I'd say its time to either flush or replace the heater core. I would flush first just to see if you had flow, and maybe even clear the sediment out. One other thing, my Metro does'nt have a heater control valve, so thats not really an issue in my case. Make sure thats not an issue with yours. (heater control valve is inline with one of the heater hoses that keep coolant from flowing through heater core) When flushing alternate the flow so as to break up sediment. Good luck.