I have the same type (RVP) and worked and worked trying to get the cover off. I ended up using a tiny flathead screwdriver and felt like I was breaking it and it did end up coming off the wall and is hanging by wires currently, but still works. I am going to replace it also. I basically shoved the screwdriver in the small crack you can see where it separates from the back mounted piece. Poking in the 4 small spots that look like it would help doesnt....they are actually clips inside holding the circuit board.
