There is no middle seat in a coupe? just a bit of removable padding between the seats. There are seatbelt anchorage plates on on either side of the transmission tunnel for the existing belts which might be modified for child seat restraints but new anchorage plates would probably be required for any upper restraints. A lightly loaded child seat might be OK in the centre but the layout is simply not practical for an adult Lap and diagonal seatbelt system IMHO. Your MB dealer would probably be able to tell you if a fitting kit exists for a central child seat but I doubt that such a thing exists?