As far as the reset...either the fuse or the battery terminal is fine. The fuse will just limit it to the circuit that it is attached to. The battery terminal will reset everything on the truck....from the clock to the system here. I would just do the battery terminal method to be sure all is reset. It doesn't explain the lack of flow through heater core. You still need to properly flush and back flush the system or at least look for blockages. This electrical stuff will not deal with the lack of flow through heater core. Need to do your best to ensure no air pockets and make sure that if you have any....they are removed.