Leo-
The thread that follows describes a few options. One requires drilling a hole through your car, but it will be covered behind the license plate. A second (and possibly better) method involves pushing up some plates in the trunk area which allows you to then to remove a brake light, and then you can probably reach the lock mechanism. See Post 14 for a picture of the plate that can get moved below the trunk. I have never done any of these, so I have no first hand experience though.

Unless the coupe is different than the sedan, removing the back seat will not be helpful. There is a metal divider that separates the passenger compartment from the trunk on my w115 sedan, so you would have to sawzall that out of the way, which is not a good solution. Good luck.

(forgot to put the link to the other thread)
http://www.peachparts.com/shopforum/vintage-mercedes/319695-locked-out-re-key-w115-trunk-lock.html

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).