I've been trying to make a couple of custom skins since the vehicle material system changed from .cs to .json, but none of them are working, and I believe it's because I've left another persistentID in there. what does the persistentID do, and where does one get the ID for a material?
They are not used at the moment, you can omit them from your hand-made material files. Also check this: https://www.beamng.com/threads/a-thing-that-we-all-need-materials-cs-json.65352/#post-1069753