Just FYI, to answer you original post edit, it simply pushes in. If you look at Roof Transport installation instructions you will see how it works.
Link
You can pop it off with a credit card. To attach it, the two pins on the bottom of the trim piece, push into the two plastic caps. To install the RTS mount, you pop it off, unscrew the plastic caps, and bolt on the RTS mount. That trim piece shouldn't come off under any normal usage.
Best of luck with the PPI!
@Nimbus, great link I have been looking for a replacement for my scratched one in the correct color!