There are a few "solutions" out there. Ideally, the best is to replace the rear glass, which has the trim bonded to it. (the trim / seal is not available separately, as it is manufactured as part of the window itself.)
The second best solution, depending upon the condition, is John Temple's solution. He's one of our vendors, and provides several add-ons for our cars. His fix is an adhesive gasket that slides beneath the existing one, smoothing it and securing it. Information, and good "before / after" shots are available in this thread:
http://www.lincolnsofdistinction.or...d.php?31602-LOD-Specials-Rear-Window-Trim-Kit
Other alternatives include "tucking in" both outside edges of the trim into the groove beneath it. If you grab the trim at the edge and lift gently, you'll see this groove. you can simply tuck the edges into that for a pretty clean look. This is an easy fix for the two outside edges, but I don't think it works for the top and bottom.
Lastly, you could remove the trim entirely. Most modern cars have omitted this already, although the Mark will leave a larger gap.. See Jeff's photos (Sierra3) in the thread below for some additional discussion on these options.
http://www.lincolnsofdistinction.or...hp?39942-Who-has-removed-the-rear-window-trim