Hello dear

I have a 2007 ml320 cdi and some time it does not start in the morning, I change the battery but the problem still there. I noticed that the brakes light is allways on when I start the car, it only turn off if i turn off the ignition. A friend conected a tester ( i dont know the name of that box) and get the message that there is a short to positive on circuit 54. this is sucking the power from the battery. What i have to do? where is this circuit 54? Should i bring it to the dealer?