Date registered: Feb 2018
Vehicle: 2000 SL R129 Final Edition brillant black
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Quoted: 40 Post(s)
There is a sensor mat built into the passenger seat bottom that detects whether the seat is occupied or not - in order to prevent firing the airbag in case of a "light" child on the seat.
I guess you can override it mechanically but you'd certainly lose your safety certificate for the car if authorities find out, also in case of an accident it is quite a bad idea because you don't get the airbag for the passenger so would not suggest to do it. You can first check the connections to the mat, perhaps it is a simple plug that is broken or open, otherwise you'd need to replace that mat, it should be available with Mercedes - depending on the model year it might be one piece with the seat heater etc so can be a little complicated, I'd guess the max cost will be approx 600$ with Mercedes.
Furthermore there is an electronic sensor in the seat that connects with an electronic sender that is in the base of an original MB children seat. This will switch the airbag off as well, even if the weight of the children seat (and the kid as well) might be higher than the threshold for the upper sensor. Anyways the effect will be that your passenger does not have an airbag.