1991 mustang convertible question
 

I am changing out the windshield and where the top meets the windshield there is some kind of weatherstrip that is deteriorating and I was wondering what this is called so I can find a replacement for it. any advice would be welcome, thanks in advance

I would check out LateModelRestoration and see if they have that part.

I believe its called the folding top front rail seal?? Click the following link & see if this is the weather stripping youre looking for.

https://www.npdlink.com/product/seal...0000%26year%3D

There is also a folding top center rail seal located on the top that runs the length of the door glass on the driver & pass side.Not sure if you need it too?? Just plug the following part numbers into the search box on the site in the link above.
M-51565-1A - driver side
M-51564-1A - pass side