You will have to purchase a new one and just replace it in entirety. I ordered mine from Fastlane (click "Buy Parts" above) and it arrived in a couple days. Replacement is very easy. Just be sure the battery is disconnected and you do not turn the ignition key on. Otherwise your ABS light will be triggered requiring a trip to the dealer to turn it off (ask me how I know).

EDIT: I meant the SRS light would be triggered. You can not reset this light with an OBDII or other simple scanner.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280