How do you glue these back on?

That may be the worlds strongest bond like is says on the package, but bonding the plastic back by melting its original material back into one another cant be beat especially in areas where you cannot add thickness like an A pillar moulding. I have used an aluminum paste version of JB Weld fixing more sections of these grille pieces shown in prev posts as well. It works well in some applications just needs a good scuffing to get it to adhere.

I have also bonded 1/16" thick polycarbonate sheet behind repaired cracks with MEK and clamps to hold it till dry. Being that the MEK melts the plastic fusing it back together to its original state its just a better bond. See pix below for bonded doubler over back side of repaired crack.

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