Working on my sons 1987 B2600. Heater quit working. Also temp gauge quit working. I replaced the thermostadt and temp quage still not working. Engine is getting up to temp though and thermostadt is opening and closing. Not getting any hot air blowing out of heater. Went out today and tried to flush heater core with garden hose. Got a little bit of water to come out one way but when I tried to backflush nothing came out. I don't know if the temperature **** is working. Fan is fine, just put a new blower motor in. Is there a valve in the heater core? Also when truck is running heater hose on passenger side is hot but hose on drivers side isn't.

 

There is no valve in the heater core loop, the heat controls in the cab adjust how much air flows through the heater core and into the cab. Sounds like the heater core is plugged up. I would squirt some purple power inside there and let it sit a few minutes then try to back flush it again.

What is the repair history of the truck? How does the inside of the radiator look. Usually something like this happens when someone replaces a water pump and uses a lot of RTV.

I doubt it helps at all but here is a coolant flow diagram, sorry I haven't added pretty colors to it yet:

http://www.mazdatruckin.com/B2600i/B26CoolantFlow.html