Rear brake light not working and fuse for rear brake light and engine room keeps blowing

I need help trying to fix this problem in my saab 9000 cd, my right brake light has stopped working originally i had thought it may have been the bulb but i have replaced all of the back ones, i then thought maybe it was the fuse, which is the one that controls the rear brake light and engine room light, and i found out that keeps blowing when i start the car or when i switch to turn my lights on, so now at night i cant see my speedometer. I've tried searching what it may be, some say it may be the rheostat, and others say may be faulty wiring. So just need some advice on what it may be and how to fix it, thanks