MB has some TSBs for installing a child seat anchor for some of their pre tether capable models. I have the TSB for my car (82 300D)
Check Alldata.com, go to "Recall and Technical Service Bulletins" and plug in your year and model. Scroll through the TSB list and if there is one and you're interested you can jot down the TSB number and title and beg your dealer to print it for you.
1982 300D Turbo