The Quarter Window Trim is supposed to be inserted in to the window seal before the trim/seal/glass is installed in to the opening as an assembly.

I have found a much easier way to do this if the seal/glass is already installed into the body (necessitated by many hours of
blood, sweat and tears, plus many *$&#^@ words ;-).

With this procedure you do not have to remove the glass or seal from the car
and once you get the hang of it, it only take a few minutes to install the
quarter window trim.

Cut a 3-4' long piece of 24-26 gauge speaker/electric wire etc. and insert
the wire into the "groove" where the trim inserts into the rubber on the
upper or lower side (one at a time). Leave some of the wire sticking out of
each end of the "groove".

Align the trim to it's proper position and press it in as far as you can
into the "groove".
Start at one end and apply firm pressure on the trim piece and pull the wire
down, or up depending on which one you are doing first (up on the upper one
and down on the lower one) and work your way in slowly in one direction (end
to end) continuing to apply pressure along the way as you pull (up or down)
on the wire.

After you do one of them , insert both of the metal corners on the inserted
trim in their proper location, then insert the wire in the other "groove" and
align the other trim while making sure you "tuck" the trim into the corners
before beginning the above process.

What the wire is doing is actually "opening" the "groove" to allow the
"teeth" of the trim piece to engage with the rubber seal.

After you do it a time or two it only takes about 10-20 mins per side.