Possible sure. If it's not legal though (pretty sure it isn't because you have to pay a license to use car brands in games as far as I know?) then it's kinda not worth the modeling time, because the models couldn't be shared.
There shouldn't be a problem. As far as I can tell. The auto industry hasn't made a big to-do about going after modders. Moreover, technically modifying a game/software does fall under First Amendment rights. Assuming you aren't doing so to circumvent copy protections (DMCA) or for profit.
Regardless, it's an IP holders legal responsibility to protect their own intellectual property. So, if they do nothing, then, in essence, do as you will.
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