We have a Brittax seat in the center of the rear seat of the 1990 300E. There were no factory attachments for the tether. I took out the first aid kit and cut a slot in the floor of the tray (It's just heavy cardboard). I passed the strap into the trunk where I slipped a 1/2 diameter steel bar through it. When it's sinched up the bar applies a load evenly across the metal framework of the package tray.
This is a solution that I'm very comfortable with, but it's just a suggestion. All disclaimers apply,...
Brittax = very nice
Experience is something you don't get until just after you need it.