When I did a lot more driving than I do now, I also did what PHJim, MikeL2, Deb Cowan, Tattie Bogle, etc "talk" about. Perhaps I should mention that, by the time I'd start practicing in the car, I already knew the tune.

When I do need to learn tune as well as words, I just play the song constantly while I'm doing other stuff, until I know the tune without having to worry about it. Knowing the tune helps me later remember the words - because they need to fit the tune.

As for learning the words, what is extremely effective for me is to write them down. (Once used to pretty much do it when I was younger; now two or three times might be needed.) If I've written down the words first, it usually cuts the time I take for memorizing by at least half.

Since I don't spend much time in the car these days, the place I currently find time to memorize/refresh words to new/old songs (mentally, not out loud; practicing the song/figuring out what key to sing in/working on how it sounds in my voice is a different kind of practice) is while I'm lying in bed waiting to fall asleep... as good as counting sheep (and more useful!).