We take 3+ 7 hour car trips a year. My kids get car sick with the ipad. We have the infotainment in our current car. You need to establish rules and stick with them.
Examples:
Only if there is luggage in the car. I know a family that lied to their kids and said it did not work unless there was luggage in the car
It does not go on until you are on the highway for the trip (or out of your home state).
We need to alternate music / an audio book with videos.
We leverage the great resources from the library - downloading audiobooks, borrowing music and DVDs.