I worked for Ford Motor almost 30 years ago, and they were testing air bags at that time. Early testing consisted of trial and error as was the case with most automotive designs of that day....most of the air bag designs consisted mainly of error. They didn't perfect them for several more years and many crash tests later. To retrofit them onto a car that didn't originally have them would be beyond an individual's capacity. Even if you could design something you thought would work, it would have to be crashed tested to know for sure.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold