In the 2017 Volt manual it's on page 40. The technical writers need to add a note on how much pressure is needed when "prying" the cap off! Yesterday I "read the manual" and went out to unlock my car by the key just so I'd know how to do it in the future. I kept thinking I was going to break something and would stop. I finally did more research online and found info that stressed you have to REALLY pull that key out, you feel like you're going to break the thing!
I'm glad I did this test open, now I know how it works. By the way, that cap does look pretty and clean, but good thing we don't have to do this often, it would break in no time. Dam plastic.
