The component in question is what is known as a clock spring mechanism. It is located in the area between the steering wheel and the steering column. It is a type of electrical connector used to allow a continuous connection to be maintained for items like the horn and airbag whilst still allowing the steering wheel to rotate. Mercedes have discovered the clock spring connection to the airbag firing mechanism may not have sufficient earthing and therefore allow a short circuit which could technically result in the airbag being deployed unexpectedly. But under these conditions it is more likely the high resistance to earth would just cause the airbag warning light to appear to let you know all is not well with the system. I would guess the clock spring mechanism is being redesigned and components have to go through production before getting into the worldwide distribution channels, hence the lag between notification and installation.Was a little confused with this part of the letter:
"As a precautionary measure we would like to add sufficient grounding to the steering components of your car. We are working to obtain the necessary parts to resolve this issue as soon as possible. As parts become available, we will contact you again to arrange for the required work to be completed."