I agree, do not waste your time with retro fitting. There is a whole lot more involved than bolting on an airbag eqpuiped steering wheel. There are a whole series of sensors that are specifically placed through out the front end. Unless you are an engineer in crash testing and the exact physics of that particular chassis, you're not going to be able to do it.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member