To get behind there you need to slide out the microwave, then you can access that area from the top. To remove the microwave you need to remove 2 or 3 screws that hold it in from the bottom. If you put your head on the chart table and look up you should see the screws in the overhang. Unscrew them and the microwave and wood panel that it's bolted to will slide forward.
Before you go through the trouble of taking it out, you may be able to use a phone in video mode with flash turned on to "look" behind there. Tilt the DC panel down. With it down you should be able to reach around behind the wood panel. I can reach the back of the VHF that way. If you put a phone in there you can probably "look around" with it.
I'm just thinking - there may be one other place to check too, and it's slightly easier to access. Our NMEA compass and autopilot computer are in the area behind the aft berth, and there's a Simnet hub there, so it would also be a somewhat convenient place for the installer to have put something like the SonicHub. It's in the area that you can get to if you're reaching from the berth to the small portlight on the transom. Our compass is on the back of the bulkhead behind the berth just to the port of that opening, and the autopilot computer and hub are in the area below the false bottom of that storage area.