First, I'd try to locate the EXACT source of the leak. It may not be necessary to replace the seal at all, just adjust it. Climb in the trunk with a flashlight, and have a (trusted) friend shut you in, and run water from a garden hose around the opening slowly. Watch and feel from inside where the water is being deposited, and check for leaks. Don't use the nozzle sprayer attachment and shoot at the opening; any car will leak under that pressure. Just run a light stream (1/4" - 3/8" diameter) around the opening. If it's not the weatherstrip, and is a body seam, etc. this will also make it evident.
Common weatherstrip leak points are where the trunk wiring harness passes the opening on the RH hinge (sometimes the harness gets pinched between the seal and deck lid), and the weatherstrip joint underneath the license plate area (weatherstrip shrinks with time, and pulls the joint apart).
If the trunk wiring harness is the problem, reposition it out of the way so that it doen't interfere with the seal when the lid is closed. Zip ties will help you here. The weatherstrip bulb should recover its shape in a few days. If it's the splice, pull the weatherstrip off the car, and give it a good stretch. You'll be able to get an inch or two of length out of it that way. Then reinstall it back onto the body flange, and use a good commercial adhesive to join the ends. Retest with water to verify the repairs.
It's possible that the bulb portion of the weatherstrip has lost it's resilience, or is damaged beyond repair, and WILL need to be replaced. If the bulb portion of the weatherstrip is torn up from dragging things over it, the strip must be replaced. I'm not sure you can get this from the dealership anymore, but a quick pass through the junk yard should net you a good one from a later-model T-Bird. In a pinch, you might find a similar profile on another make of car. Just make sure that the overall dimensions are the same, and it's long enough to cut and splice in the factory location. Again, retest with water to verify.
Good Luck,
JD