I just broke my little plastic piece too - and I've mended it just as easily. Thank God, because my car's out of warranty.
It's so easy to break - Whilst driving along I'd locked the car doors internally (because I'd upset another motorist with my appalling driving technique and I was worried he might attack me), then I got out to fill up with petrol (or gas as you call it) and I couldn't understand why the filler flap wouldn't open. Apparently the car lets you get out, but it still keeps the filler flap locked.
I was puzzled as to why it wouldn't open (didn't connect the internal locking with the problem). I got impatient and pulled the filler flap upwards. Of course, the little plastic locking piece broke.
Luckily I saw the little plastic piece and kept it.
When I got home I simply removed the filler flap lock assembly from inside the boot (or trunk as you call it), drilled two small holes - one in the little broken plastic piece and another in the end it had broken off from and connected them back together with a small double-ended screw (ebay).
...or you can pay $95+shipping for a new part!
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'03 Grey >>> '06 Black >>>'10 Silver >>> '16 Grey ^
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