I think what you found was a tuner box. The in-dash unit is just a remote for that tuner in the trunk. If it's like my 1991 126 model...the amps are behind the back of the back seat back. You have to take the seat bottom out, then there are 3 screws to take out the back. One screw on each bottom side and one screw in the bottom middle. But, I am thinking if you take the fuse out of the tuner then the amps will be go unpowered. I just dont know.

Other than that you can disconnect the antenna power in the trunk on the drivers side in case that sticks on. Other than that I just dont know. In my case I had a digital meter and did what you did and everything checked out even the battery. So an MB mechanic gave me a hint that it might be that screwy setup on the radio system....he was right. That was a 400E and the amps were separate (2 of them) behind the trunk panel along with the tuner. Did you take out the fuses that dont go thru the fuse block...one of them is the trunk tuner and I think the radio head in the dash is fused, too. It took me a month of checking things out before I spoke to a mechanic friend and found that bad amp.

Until you find out what is causing the drain you can just unhook the battery like I did and rehook it when you want to drive it.