I have a Sovreign, Model I, Serial # 492010, a 1989, I think. The heater keeps tripping. I removed the heater, found that there was a kink in the thermostat tube leading from the heater to the "black Box,"and there was very small leak. I assume that this is what the problem was, as the heating element seemed to be OK. My real question is... how do I get into the black box to remove the old thermostat and replace it? I can't figure out how to access the box, without tearing apart the wooden skirt. (by the way, what's the black box called?) There are four screws on the back side of the box, but there is no clearance to reach them, and they do not look like they are what is keeping the box secured to the skirt.

The black box is the control box. You should be able to take out the 4 screws on the back of the box to access it.

Thanks for the response, mmelvin. Is there some way to detach the black box from the outside wooden skirt/wall? It is such a tight squeeze to get the four screws out. The only way I could find the screws was with a mirror and a flashlight. I'm pretty sure that I would not be able to remove the old thermostat assembly from the box in the position it is in.
Thanks again.

To access the screws to the black box you need to remove the front control panel. The screws are behind that.