The dealer can check the roll bar system by plugging the hand-held-tester to the 16 pins connector under the hood (passenger's side).
The light in the switch is flashing to indicate that the roll bar will not come up in case of an accident. Your manual probably tells you to have it checked at the dealer (they do that by plugging the car to the compu).

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)