Seatbelt securing works very well. There's a blue plastic and steel winch like device we use to secure the babyseats in our cars. You can get it at Babies R Us. In my (former) 94 E500, we were hit head on by an out of control Toyota Sequoia. My (then 3 year old) son's seat didn't budge.

__________________
I never go far without a little Big Star

1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k