I thought the front airbags require a certain degree of almost direct frontal damage before they blow up. The sensors should be located somewhere behind the front bumper area on the chassis. The same should apply for the side airbags which only just came into fashion a few years ago. Rear impacts should not trigger the front airbags as they are meant to protect from frontal knocks and protect the driver and front passenger from smashing their faces into the dash and windscreen. This is only a layman's opinion by the way, thank goodness you walked away without any major injury! That is why we drive the Benzs, they are built like tanks!