Before I had my heater core replaced, the tech said they disconnected the electrical connector to the water valve, then ran the car to see if heat then came out of the vents. It didn't work in my case and the core was replaced under warranty.
I have no idea what the valve body looks like or how to get to it. But if you can find it and disconnect it, that may prove if that's all you need. Sorry for the wild-ass-guess.