From Alldata:


1 Withdraw ignition/glow start key
2.1 Remove rear seat
2.2 Fold rear seat cushion forward and remove battery compartment cover
2.3 Fold trunk cover up
2.4 Remove cargo space cover
2.5 Remove battery cover in right side of trunk
3 Connect quiescent current circuit retention unit
4 Unscrew screw (1) and remove holder (2)
5 Unscrew nut (7) and remove strap (8)
6 Disconnect ground cable (3) at battery
7 Disconnect positive cable (4)
8 Detach vent pipe (5) at battery (G1)
9 Remove battery (G1)
10 Reinstall in opposite order

Notice #3 above. I checked my owner's manual (98 c230) but it didn't give us a clue. Reading the above procedure, it appears that MB expects some latent power to be supplied when the battery is disconnected, likely to maintain computer memory.

In the Owner's Manual, it says to contact your dealer or call the 800- HEEELLLLPPPPP! number.