Electrical problems can be very elusive to track down and it would be very difficult for service to give a time estimate without knowing which component is being replaced.

As frustrating and nerve racking it is I would proceed with the repair, however I would quiz the service manager about the qualifications of the tech who will do the work. I would want a tech who is knowledgeable in the M-Class and is the dealer's best electrical person. Then I would work at some type of guarentee of no, or reduced, cost if the problem is not solved.

Hope they find your problem quickly.

Jim W.