I just had a similar problem and have just resolved it. Although im not sure how. The plug light had been on for a few days and I finally got to my mechanic today. It came up as the brake switch. Bought a new one and fitted it but still no joy. So started to hunt around the light cluster onthe drivers side to see if any thing was hanging out. Still no luck. Check the bulb, all fine. Then checked the fuse (no.9 to row 10amp) that had blown. Replaced that and it blew straight away. Then took the pasenger side cluster out and checked it. Refitted a fuse and removed the two light clusters to try and eliminate them one by one.
The middle brake light now worked and the plug light went out. Plugged the left cluster back in, that worked, then the right, that worked!. So I found no earthing out on the body and could not really find the fault but they work now???