yeah its a chair and you ride around while it plays nyant cat
There are for both. The FO3 car mod just gave you the new car from Tranquility Lane, so it looked a bit out of place, the one for New Vegas gives you a convincingly old and beat up Highwayman to drive around. I don't know how game breaking either is, but they both function - given the rubble and cliffs in Fallout 3 a car isn't really that useful, however. Google can find both for you easily enough.
I honestly think they should have put a working motorcycle in both of them as a reward for a quest or just killing the guy owning it