It was a car. The song specifically sites "three little girls, sitting in the back seat, all of them in love with Fred! I said why don't one of you come up and sit beside me? And this is what the pretty girls said:

Keep your mind on your driving, keep your hands on the wheel
Keep your kookie(?) eyes on the road ahead!
We're having fun,
Sittin' in the back seat
Kissin' an' a-hugging with Fred!

'Least that's how I remember it!!

A