FIRST OF ALL - MEASURE! There is a German and an American Mercedes and they differ greatly by like 7 1/2" different! The Import American Mercedes has an opening width of 30" wide.
The German Mercedes (gray market) has an opening width of 37 1/2"
Measure that seal before doing what I did, pull it mostly out - only to notice that I had more more old rubber than new rubber.

As for the seal going into the slot, I found it went in like a charm, of course WD40 sprayed on both mating surfaces makes a world of difference. (learned from working in shop where we mounted Lexan into rubber molding that mounted into steel opening)

Which Brings me to my own Question - Where to buy the Seal for the 123 240D ? (gray market)

The MB Dealer (parts manager) in Okemos, MI told me he could order from Germany any part. That just plainly sounds expensive!