When we have the grandson in our 98 C230, we put the car seat in the middle of the back seat. When he was younger it was the rear facing, then forward facing. They have gone in the middle with no problem. (But his Graco turbo booster needs to be to the right or left of the center, as it relies on a shopulder harness system.) We use an upholstery protector rubber mat from babies-r-us under the seat. With the seat in position one of us leans over the seat and pushes down firmly on the seat, while the other person pulls the center seat belt as tight as possible. It can be done by one person, but is easier with two people. When cinched down, the baby seat feels like part of the car - no wiggle or movement at all. Don't leave the seat cinched down tight like thatr when not in use - loosen it up a bit when baby isn't on board - or you might deform the upholstery and padding over time. I don't know why your baby seat would be so loose!

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John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E