I have a 2001 ford mustang I recently bought a Roush body kit for it off Facebook and when I installed it, everything fit perfectly except the rear valences (rear bumper pieces) the corners of them won't stick i used 3m adhesive promoter and 3m double sided body tape to install all the pieces. It's not a big deal as they're solid and don't move but they don't look good.
So I was wondering if anyone had any suggestions on how to fix it?
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https://lib.americanmuscle.com/files/69011.pdf

Complete installation instructions. As long as you you have a STOCK rear bumper
cover, it should work.

You will need the scotch brite pad, acetone, adhesion promoter, and 3M tape.