One of my door window seals seems to have shrunk a bit. I have some of the old seal in the leftovers box so i'm looking at sticking a small piece onto it to extend it back to fit properly.

What is the best adhesive to use?

Contact cement, put the adhesive on both parts, let it dry a bit, carefully align the parts before they touch and put them together

The shrinkahe you speak of is a result of pulling the seal through the track. Rubber has memory, and even though you think you've compressed it back, you never really get it all. So over time with expansion and contraction, it returned to it's unstressed size. This is why Ill tell anyone that instalks tbe targa to window seal to locate where the end shoukd be and install the outside edge 1st pushing the inside edge in to secure it.

With that being said, you can use super glue for a good bond.