Contact Cement that you can buy at the hardware store. Brush a coat on the trunk seal area, and a coat on the flat area of the weatherstrip to be sealed, let air dry for 10 minutes and then stick them together. One time shot, so have it pre laid out around the trunk opening so all the lengths come out right and be careful about stretching the rubber so it does not come out too long.
Can clean up the contact cement with a fast dry enamel reducer.
Contact Cement is pretty much the same thing as 3M's Weatherstripping Adhesive . . . and for a lot less money.