I found several class and vehicle skin upgrades/customizations/unlocks, e.g.:
Do they carry between games? Do some always drop in the same locations? Do they always appear in the same places or after the same quests? If so, what and where are all of them? What do I need to know to collect the whole list?
Best Answer
Most customization rewards are for your current class, but you can pick up/activate other classes' customization items. Skin collection progress carries between characters. Activated skins show as "Already Unlocked."
This wiki is a work in progress—modify the listings if a skin isn't class-specific.
Mission rewards
Challenge rewards
Misc.