DWStiles

I have a nice basket-style steel roof rack ordered but it looks like the maker is not going to be able to meet the promised schedule so I'm looking at alternatives for my upcoming trip.
I picked up a couple extra movable cross bars at the pik'n-pull but on both vehicles the plastic end caps on the mounting rails were broken off so it was easy to slide them out and off.
I figure with 4cross bars there will be enough support for a ladder and a few tools too long to fit inside.
what is the trick to getting the end caps off with out damaging them so that they can be reinstalled securely . I removed a screw but there seems to be something else keeping it from sliding free.

EDIT: Nevvamind, got a flashlight and a large magnifying glass and looked in and front the little plastic tit on the bottom that needed to be pressed down. got all four bars on now. went along the bar and tightened all the screws into the roof though. about 1/3 of them were 1/2 turn loose.