According to the wiring diagram the wire colors should be Black/yellow/red on pin #2 - 81a, wire grey/yellow should be on pin #1 - 81, wire brown/black pin #3 - 50, brown pin #4 - 50.
Pin #4 goes straigth to ground. When hooked to the brown/black wire the starter relay is energized.
Pin #2 comes from the fuse and hooks to the back up lights through the grey/yellow wire on pin #1.
Bosch Master, ASE Master, L1
33 years MB technician