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Dave, the GM bra has some rubber grommets that you install in the fender well, and then you screw the snaps into those. It's easier to just get some 5/8" #8 marine snaps from a marine supply store or auto upholstery shop and use those instead.
This has been beat to death, but if you go with a GM bra, pickup 8 (that
will give you a couple spares) #8 marine screw snaps (5/8" length) at
a boat marine dealer or I've heard Home Depot has them too. Just use
the marine screws in the fender wells instead of the ones that come with
the bra, much easier to install!