But will a 10-12 faux gas cap fit assy onto the black out panel of a 13-14?
i want to replace the GT with a pony or custom cap.
That may depend on how you define "fit."
Excuse the length of what follows, I want to be clear:
I have a 2010-12 faux gas cap, and a complete 2013-14 decklid panel, both bought on eBay, both uninstalled. I originally wanted to replace my GT emblem with the tri-bar pony, but later decided I wanted the '13 panel instead (although I'll paint it flat black).
The '10-12 faux cap appears to have three pieces: the base/outer ring with a brushed chrome finish; the inner ring with a bright chrome finish; and the center disc that has the emblem itself on a black background. I say "appears" because these pieces are fused together with one-time-use pins that will have to be broken to take them apart, and I've not yet done that.
The '13-14 panel is similar, except that the brushed chrome base/outer ring is replaced by the large gloss black panel.
The bright chrome ring and the center disc are the exact same diameter, and have exactly the same profile, on both versions. But looking at the back sides, the number of pins and their locations are different.
My best guess, then, is that although the pins and holes will not match up and allow the pieces to snap together like they do at the factory, it should be possible to glue them together with epoxy or double-stick tape and never know the difference.