One thing to add is that you will often be able to bring a car for free on a southbound trip, while you will have to pay go north, since that is much more popular.
Get a car onboard for free, how do you get that?
If you are going to have a car onboard, remeber that they take your key, and maybe drive it on shore or move it. I don't like that, so I tell them to not do it.
They do not know how your car is operated, so if you are like me, tell them to call you if it is a problem on the car deck.