What you're talking about is called "Syncing." To do this, you'd set up whatever device you're using in the car (I'm guessing a USB stick or something) as a device in MB, set the syncing rules to put everything in the same folder on that device, and then sync the playlist to the device. The playlist will be copied over, but so will all of the files in the play list. When you change the playlist, the files that you add to it will be added and the files that you remove from it will be deleted (with, I think, confirmation).
EDIT: Alternately, you can right-click the playlist, do a "Send To" -> Folder (Copy) -> Copy files to folder and send them to the drive that way (assuming that's what you're using, you didn't say), but this will not delete things you remove from the playlist from the drive as syncing would.