I'm currently working on some configs for myself that may or may not be released, and I was wondering how to write an info.json file. I haven't seen this anywhere on the wiki so I figured this would be the best place to ask.
The good news for you is that they are fairly simple info.json = Some metadata about your car such as body style, name, author and all availible stock paint colours. info_[varient of vehicle].json = Some metadata about a particular variant, such as vehicle cost, drivetrain etc. Just open any of the official vehicles, copy their info.json file to your own mod and change the values (in any text editor) to be suitable for your car. Same goes for the variant json files.