I don't think this will be possible, there's no space between the interior trim and the body. Plus the shape of the airbag is designed to fit the car, they aren't universal.

You might be better off trying to retrofit seat mounted thorax airbags. Those seem to be more "standard".

A bigger problem is the calibration of the crash sensors. You can't just take the whole system from a Jetta and plunk it into an S Class. I'd be worried the system would fire too early or too late, and there's no easy way to test it.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.