Interesting question there, Massimo. Please allow me to pass on some facts on the series 4 Spider airbags. We own two Spiders, a '91 and a '94.
The airbag systems in these cars were a nuisance, a continual problem. The most common problem was if you hit a pothole or fairly large bump in the road surface, one of the sensors would engage, thus illuminating the "CHECK AIRBAG" warning light. Unless you knew how to reset the airbag processor unit, you had to take the car back to the dealer for a reset. This problem was fairly common. Our airbags are unplugged at the steering wheel, with the connector taped. I got very tired of dealing with this situation!
The airbag system itself is quite complex, consisting of a small dedicated ECU, several impact sensors, wiring with special connectors, and of course the airbag unit itself within the North American steering wheel assembly.
I suggest you consider the risk of attempting to integrate all this into your Spider successfully, knowing that about any type of short circuit or electrical fault could possibly either disable the system or even fire the airbag. Would you really want to risk that happening while you are driving? With all due respect, I hope you reconsider (although I understand you have a concern for safety). Airbags are engineered into modern vehicles, not added onto them.
Regards--
