I just rebuilt one of my cars' driver side lock cylinders a couple weeks ago, so this is fresh in my head.

When you remove the door handle and subsequently the lock cylinder, you will notice a black plastic component that clips on and "wraps" around the cylinder - this is the alarm sensor. A green wire comes off this component, runs through the door, and ties in somehow with the security system. Interestingly, like I said the sensor just wraps around the cylinder but is not mechanically activated so I guess that means it is an inductive sensor. Knowing that puts things in perspective as it is very easy for this sensor to lose sensitivity with age and exposure to elements.

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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