Hello,
I have installed these pieces before. It's a very difficult trim piece to install. How you have the trim piece in relation to where the drip rail steps up; you need to slide it back a little.
Match the curved part of the trim to the curved part of the body and it will be positioned correctly. Some of that flat piece will be between the body and the stainless piece, touching the body.
Just keep in mind, the back part of that moulding that curves has to be sitting on the curved part on the body. So, you might have a little space between the trim piece and the roof rail step.
What I do when I install the piece is to go in on a 45 degree angle at the same time push up hard on the bottom flat side of the trim and it should slip over and in.
What you do next is put the long roof rail trim up against the roof rail on the body, follow it forward and check where it meets the a-pillar side where the angled trim piece is installed .
Now you can see how the roof rail trim piece covers that rear trim and the downward positioned a-post trim.
I hope this helps you.