I feel for you :
I have a thread here, do a search "300sd trunk lock failure" and there are some pictures on a related thread. I now understand how that mechanism works in a 126, but it's hard (for me) to put into words - so I took lots of pictures and will share with you if you get stuck just PM me.
You may read a thread about a "magic" cable under the back seat - it does not exist (on the 126). Entry through the first aid kit and license plate light are not options either.