Barry, are you sure you are in the right socket. You should be testing socket # 6. If the light is still on you should verify the circuit. I would have to look at a diagram which I don't have available, but it is my impression that the light will be on with the crash sensor unplugged. The light is turned off by the crash sensor putting batt voltage on the grounded side of the bulb (I could be wrong).
The headrests should be a separate issue.
Bosch Master, ASE Master, L1
33 years MB technician