Well, from your location it seems you are not likely subject to ice causing the whole trunk to stick to the gasket. So, it might be the lock. Just to be sure, insert the key with the squared black plastic head on it, turn it all the way clockwise, then all the way counter clockwise. It should stop just slightly before the key/keyhole is vertical in one direction. With the key held against the stop in the direction where it is off vertical, push the cylinder/button in and see if the lock opens.

If you have already tried this, I would try it again, and remember to hold the key against the stop while you do it. Let us know if that worked or not. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)