you need both, trust me.
I got the rubber i needed thinking I could just push it in and be done. well it fit but was all bumpy and looked bad. so I figured I would get the second part and do it right, then I figured out the long process uggg
but there is so much involved I have the parts sitting in my shed.
you need to take off both sides of the door rubber, and the A pillar, 10 bolts each side, then pull the old retainer out, and scrape the old glue out, then lube it with dishsoap, then insert the retainer, etc. etc.
way to much crap to do to replace it. I left mine off. cant even tell its gone.
took it to the dealer they said they dont do it anymore they refered me to a windsheild glass place to do it.
i took it to that glass car windsheild guy and he thought it was a new corvette, I left and gave up.
save your money.