92 400E Battery drain

[davidlee]

I recently purchased a 1992 400E
and its having a battery drain issue. I have pulled every fuse one by one along with other relays located behind the fuse block and tested the negative battery cable and my tester light lights up. I also pulled the fuel pump relay and unplugged the power antenna with the same result. there's a silver box on the passenger side underneath the the hood up against the windshield with a few what looked to be control modules inside of it and then one located outside the box behind it.
I didn't touch the ones in the box but I unplugged the one outside of the box and still got a power reading on the negative cable.
I read these cars have a biodegradable engine wiring harness, could that be to blame? and if so is there a way to test.
I also wanted to add that it had this problem with the battery I purchased it with so I bought a new battery, still has the same problem