Sorry to hear about this. I just went through the same frustration.

I had suggestions from driving a small screw driver into lock and destroying it and it will open, also can drill hole through chrome peice and insert welding rod and open, also go to lock smith and he could not get it open, also go to MBZ dealer and they will drill out old lock and replace with new keyed lock to match others for $200 plus. Not liking any of the above I started thinking like I did in many days ago when I was a Millwright for a living.

I went under the trunk and measured from appox. the center of the spare tire hold down bolt out to about 12 inches and drew a circle. I then drilled a starter hole and used my jigsaw to cut a 12" diameter hole in trunk bottom. I was able to push the spare tire up and out of the way enough to get to the lock and open the trunk. Worked on lock and now it is working. Now I am not sure if I want to repair the bottom of trunk incase it happens again. Maybe leave it for a oversized drain hole.

This may all depend on what are in your packages as well.

Hope it all works out for you.