Latch is the bars located between the seat cushions. The top tether bolt is carried by all Canadian dealers. Unfortunately, mine are installed and I can't see the part #. You might have some luck calling MBUSA.
There are three slots for the tether bolt. Under the rear headrests on each side, and one in the center by the first aid kit door. You have to cut the upholstery, but no biggie.
On MB cars you should make sure the bolt, if using only one, is either installed in the center (if your seat can use lap belt alone) or on the driver's side. Many people don't use that position thanks to the advice of "professionals." They don't have a Mercedes. Not sure about the US, but in Canada, side impact beams are mandatory in passenger side doors only. This is due tot he much higher rate of side impact on the passenger side. Therefore, it would seem wise to put your child on the driver's side. But, most cars don't have extra steel beams in those doors. BUT, a Mercedes does.
1998 C230 "Black Betty"