no reverse sel 420 fix it
The clutch pack that engages for reverse is the 'front' one in the transmission. In all probability, the lip seal around the input shaft has cracked, allowing the apply pressure to leak out.
Since this is the first one behind the pump, the tranny does not have to be completely disassembled.
I has this done to mine from a 89 560 SEL. The tranny work, including parts was only $150, and done by a shop that does all the MBZ and BMW trannys here in Houston (and authorized to put the factory Remanufactured stamp on them!
Replacement clutch disks and seals are inexpensive ($25).
The tranny removal/reinstallation labor will likely cost $300-$400
No need to rebuild the entire tranny.