rumble seat

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An uncovered passenger seat that opens out from the rear of an automobile.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun dated An extra passenger seat or row of seats in an automobile, which typically folds away into a rear storage compartment.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a folding outside seat in the back of some early cars