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The switch is in the left hand latch mechanism, and can be accessed by removing the shroud. (C1PL25)
They are a common failure point. If removing the latch to clean/check/replace it is too much effort, or you need a quick way to lock the car, you can just dead short that connector out with a piece of wire pushed into the connector.
Apologies, my message was a bit confusing there.
C1PL25 is the connector. It is plugged into the right hand side of the bonnet/hood latch mechanism where the tip of the arrow is.
You need to remove the plastic cover from the front of the engine bay (they are fitted with torx screws on the X351) then you can access the connector to remove it.
Then short between the two female pins on the connector to make the car believe the switch is closed.
Once you have exposed the latch it is worth checking to see if it is just stuck, and that the connector itself was securely fitted.