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Date registered: Oct 2010
Vehicle: 1992 600SEL
Location: Melbourne, Australia
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My 91 300SEL had this switch but it also didn't work, and yours is a post-facelift model so it may be different, but here is what I know.
Cars with this feature have an extra vacuum motor in the trunk a few inches to the left of the lock. This is connected to a cam inside the lock via a spring loaded rod. The motor is connected to the same vacuum line as the one that unlocks the trunk but the spring on it is much stronger. When the PSE pump operates for a short time it unlocks first as it has a lighter spring. If the pump continues for a few more seconds enough vacuum is developed to pull the second motor and turn the cam, popping the trunk.
When the switch is pushed the convenience module should activate the PSE pump for about 5 seconds to unlock/pop the trunk.
Assuming your car is the same, can you hear the PSE pump running under the back seat when you press the switch?
Also, when you point the remote at the trunk sensor and press the button twice, does the lid open?