Try getting the service CD on Ebay. It has the entire wiring diagram and you can track each circuit behind each fuse.
It helped me with a drain that turned out to be the trunk light remaining on. It was because I removed the panelling that the switch pushed against and would not turn off the light.
Good luck!
'87 300D (178km)
'01 CLK 430 Convertible (21km)
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