The key that works the passenger door and glove box should work in the trunk. It sounds like the starter switch has been replaced and the driver's door lock vandalized. Squirt some WD-40 or some other penetrant into the trunk lock slot and all over the key. Push the button in and out to make sure it is free to operate as well. Then, slip the key into the hole and use a little force in each direction. Repeat the WD-40 treatment, push the button in a few times and then try the key again. Make sure you get it into the hole up to the stop. it is entirely possible the lock is just full of crud and is sticking. This can make it difficult to turn at all, even when the key is all the way in. Also, do the vacuum locks (central locking feature) work at all? If the engine starts and the battery is charged the vacuum pump should run the locks and unless someone specifically locked the trunk out of the system, it will unlock when the locks are cycled. Which should work by sitting in the car and pushing the driver's button down and then, once all the doors lock, pull the button back up. All the doors should unlock, including the fuel filler flap and the trunk.

Give it a shot and let us know. Mining your way in through the back seat might also be possible, but only as a last resort. Jim

__________________
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)