Just picked up the car yesterday and the last owner must not of used the roof rack in a while. the key seems to be a plastic spline tool, but seems like its going to break before it unlocks. looking for the plastic end caps for cross bars also

I'm not familiar with these. Is the tool plastic or the fastener on the rack plastic?

I know some vehicles use a plastic tool with a torque clicker built in to prevent over tightening. Usually this can be matched up with a metal driver of the same size to use for removal. The trick is determining which type of drive the tool is. (Double hex 12 point, 12 point square, 12 point spline, torx, etc)

Can you post a picture?  I only recall using a T40 torx to do roof rack installs but its been a little while since I had one off.