The seam that runs down each side is manufactured into the material.
This is the "wide seam" version of the top. Where the seams run closer to the edges of the roof. This top was originally designed for the SE formal roof only. The other models were to get the top with a more narrow seam.
But in practice on the factory floor they just seemed to have fitted what ever vinal top was to hand. So your Challenger could have come with either.
I have had a narrow seam one fitted in the past. I thought fitting a wide seam version would make a change.
I did have to make one seam though. I made a pleat right in the middle of the thin bit that runs under the rear window. I couldn't get it to fit any other way. I am sure the seams are supposed to run down the outside of the sail panels more. But I couldn't get it to stretch out that way.
It's the best I could do and it don't look too bad. It sure isn't going to get re-done!