With one character at level 50, having unlocked quite a number of different vehicle parts, all of the red-highlighted place-markers for vehicle parts in the docking bay still don’t do anything.
Each character has to unlock the vehicle parts seperately. The red ones are from the hijack challenges. Look in the challenge section on each map to see if that map has a hijack challenge. If it does you find the purple vehicle on your map and go steal it.
To unlock parts that aren’t attached yo challenges, you simply hijack random enemy vehicles snd turn them in to discover if you earn new parts for the catch-a-ride station.