Assuming that the word was new-ish, I failed to check Partridge, who says,
rubber-neck; necking. A very inquisitive person; excessive curiosity or inquisitiveness U.S. (-1900), partly anglicised, esp. in Australia, ca 1905; slightly ob. Ex 'considering craning and stretching', as though one's neck were made of rubber: as in The Pall Mall Gazette, March 8, 1902.
rubber-neck, v.i. in the sense of the n. (q.v.): 1932, Dorothy Sayers, Have His Carcase, "She...could not waste time rubber-necking round Wilvercombe with Lord Peter [Wimsey]."
rubberneck car. An observation car: Canadian railroadmen's (-1931)
So what do we have? An American noun--still no trace of point of origin, though the date range is very narrow--that quickly spreads elsewhere in the English speaking world. Only later does it become a verb, referring to two different kinds of gawper: the aimless tourist and the accident vulture.