What goes in there?

I have a 99 National M6350. On the post (narrow wall just outside the bathroom) where the heat/ac, tank levels, and the CO Detector are located, there is a rectangular hole at the bottom of the post (narrow wall just outside the bathroom) that is about 2 inches up from the floor. It is the same size as a electric box and there is a green wire and a red wire inside the box.

My question is, what is supposed to go in that hole?

Any help will be greatly appreciated.

Thank you ALL in advance.

John