Depends on if you want the original style (repros can be hard/stiff and seal poorly0 or a softer alternate.
The seal goes on the trunk lid. I find it easiest to apply with the lid off the car.
If original the "C" shape points with the opening inward. See this site (one of many that posts the same generic pic -
http://www.mustangdepot.com/OnLineCatalog/Rubber/trunk.htm)
Most vendors sell this version.
I found for the fiberglass lid the 69 (IIRC) style seal works better. Not correct, but far softer and provides and easier seal. Its a trapezoidal with 3 hollow "tubes" inside. Nice and soft and squishy...
I bought mine for Tony Branda, not sure who else may sell it.
http://www.cobranda.com/trlidwe.html I think it was the 69-70 version I used, they have no pics of the sections, and just don't recall off hand...
Whichever you go with... I find the middle of the seal and glue from the front center of the lid around to the latch. Prefit to be sure its long enough (most have a lot of spare length, so no issue there). When I'm about a foot from the end I lap the two ends over each other, make a mark above the latch at the center, and cut one end square. Glue that in, then run the other end and cut it about 1/8" long, glue down and glue the two ends together. The extra gives a good solid contact to the ends.
I use 3M weather-stripping adhesive. In order to hold the seal in place you can use masking tape or painters tape at intervals as required. if you apply the sealant right its generally not needed, can be helpful if you are doing the seal with the lid on the car as that gets awkward...
HTH, good luck!
