I know this is a long shot, but my 16 yr old son has an 87 Mazda B2000 and he can't get the heater core to put out heat. He has flushed the core and the water flows through it fine. He replaced the engine thermostast and that seems to be working fine, so I suspect the control valve is not letting warm coolant into the core. Where is the heater control valve on this thing? My son absolutely cannot afford to take it to a shop. Thanks.