Stealth

I believe it is held on by strong 3m adhesive, plus two screws/bolts.

One Bad Stang

Does this happen to all of them? I have never noticed this after a lot of washes and rain.

157dB

The 'dingus' is not flat on the back nor a solid piece of plastic.
It is a hollow structure and has a few plastic ribs for strength.
The 3M tape its attached with only has minimal surface
area to attach the 'dingus' to the trunk lid. It is 'aligned'
with two 'alignment pins' and there are not two bolts/screws
as one would believe. You cannot even access the rear on the
badge from inside the trunk to use any attaching hardware.
The 'dingus' only has a thin lip around the perimeter on
the back to seal the rear cavity from water intrusion.
I pulled mine off and it was barely attached (good for me,
the 3M tape came right off, bad for water intrusion protection)
around the perimeter lip.
If they were going to put a 6" round gas tank filler medallion
on there, they could of at least made it forkin functional as
a fuel filler, not a fricken water/dirt/rust attractor.
Thats why mine has been removed and replaced with a panel
sans dingus/dingus hole.