The problem has to be the trunk lock. Mine works just fine. I haven't taken that area apart. I suspect the trunk key must unlock the trunk via the central locking system as opposed to the door key mechanism which pulls up on the locking mechanism and causes the other doors to lock. My locking mechanism is getting sluggish so I suspect I have a small vacuum leak somewhere but other than the drivers side door key not turning mine works great. Good luck. I imagine there will be an easy solution. The worst case is you have to get a salvaged trunk lock and have to carry around an extra key.

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My Daily : 96 E-300 Diesel with 195,000 miles
Retired: 92 300D 2.5 T 345K miles and for sale
Retired: 95 E320 157K miles and currently parked with blown engine

Both retired cars are for sale as is my w124 shop inventory