I'm not sure this will make you feel any better, but I had an interesting experience today kind of related to your question.
I have a 94 S500, and the SRS dashlight would remain on for exactly 2 minutes EVERY time I started the car.
I had inquired here about it before, and was told it was probably the seat belt buckle, so I at least had a heads-up on what was wrong.
Anyway, took it to the dealer, he diagnoses it with the scan tool, and calls me to let me know it will cost $495 to replace the buckle (and sensor, I presume). Since the actual seatbelt functioned fine and the only problem is the light for 2 minutes, I declined and said I'll be by to pick it up later. Paid the $95 diagnostic fee, and was on my way.
Well, made a number of stops on the way home, and would you believe the light goes out 4 seconds after startup, just like it is supposed to. I have NO idea what they did, but it appears to be fixed. And yet, according to the diagnostic printout, it still needs a new switch.