I left my Mercedes Benz in a parking lot this past weekend since I was going out of town. It was left in park, the radio was off, and the lights were off. When I got back, I put the key in and it wouldn’t even turn to start to car, the radio wouldn’t turn on, and the horn wouldn’t honk. I tried to jump the car but it did nothing. What is wrong?
Your computer works 24/7 waiting for a START command. In your case more than the computer is working overtime.
The way you troubleshoot is temporarily replace one of the battery cables with a much lighter wire. Now all current flow is thru this wire, and any time current flows thru a wire, a voltage drop is observed. The higher the current, the greater the voltage drop.
Set your digital Volt meter to its lowest DC voltage scale. Attach your meter Leads to a point close to either end of the wire you installed. You'll see a voltage displayed because the computer along with the unknown device(s) are drawing current. So now start disconnecting fuses. If no change is seen on the meter, go on to the next fuse. Don't be surprised if you find more than one additional device drawing current.