It would be a serious mistake contacting the car companies.
I don't think you want to be on their "radar".
Most likely, they would say thank you if you notified them.
However, you never know if they might want to hassle you over it.
If you want to transfer them to the car companies without them knowing it.
You could move them into a separate acct. And, copy their data from their
primary whois. Also, point the domains to the car companies site.
And, then they can scratch their heads wondering how they got them. When,
they get notifications of renewal.