It seems to me as though you understand the separate trunk locking. Either way, I will try and give you adequate directions. I'll go down right now and see which way locks it!
Ok, putting the key in and turning it to the left (about 45 degrees) unlocks the doors and trunk. Turning the key to the right unlocks the doors (again, about 45 degrees).
Ok, now if you put the key in and turn it to the right, the doors lock, turn it again (about 45 MORE degrees), then the trunk is locked separately, as long as you pull the key out when it's 90 degrees to the right.
To sum it up, if the keyhole is in the 'normal,' vertical position, the trunk should open if the car is unlocked. But if the keyhole is horizontal, then the trunk is locked individually and will not open when the car is unlocked. (Separate locking of trunk doesn't allow valet key to work in it.)
This might have been a waste, but give it a try and let me know what you come up with!
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)