This is very interesting. I am planning to do this very same upgrade for transporting my daughter in our older Mercedes. No matter how much I crush the baby seat into the factory seat and pull the belts tight, by the time we reach our destination, her seat is loose.

i like the idea that the gentlemen used in the link above, I dont know that the eye bolts are strong enough, I plan to use a "U" bolt. I am not crazy about the small washers in that example either. The "U" bolt option requires 2 holes drilled so I feel it best to use a plate to back it up. That should be plenty strong.

No comment on the top mount. I will later share what I do for that. In our 300SDL and E300, I would use the same method as the bottom, through the steel of the hat shelf. In the 300TDT, I have to see what possibilities exist.

Why would someone put effort into adding seat belts when direct clip is the best option?

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83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.