It's called a "tonneau flap cable". Getting hard to find. With all due respect to ala_xfire, I don't know of anyone who has been successful having the cable repaired. Suggest you look here:

https://www.ebay.com/itm/BRAND-NEW-G...53.m1438.l2649

or

You can sometimes find the tonneau flap(s) being sold WITH with the cable(s) . . .

https://www.ebay.com/sch/i.html?_fro...+flap&_sacat=0