In a Kipling story, guests have arrived in early model
automobile. A child riding in uncovered front seat is chilled.
"It's quite warm in the tonneau. Why did you let him ride in front? "

Webster's 1913 Dictionary

Definition: \Ton`neau"\, n.; pl. {Tonneaux}. [F.]
1. In France, a light-wheeled vehicle with square or rounded
body and rear entrance.

2. (Automobiles) Orig., the after part of the body with
entrance at the rear (as in vehicle in def. 1); now, one
with sides closing in the seat or seats and entered by a
door usually at the side, also, the entire body of an
automobile having such an after part.

3. = {Tonne}.

Here's a picture of an antique auto, showing uncovered front seat, and back seat with cover folded down, which
could be raised to protect passengers in back seat.
hhttp://motionalmemories.com/1904-CGV-25hp-Roi-des-Belges.htm

I've seen tonneau used for the cover on the back of a pickup truck, usually a soft vinyl material. not to be confused with a hard shell cap.

A tonneau is a wooden barrel in French. I'm wondering if it isn't related to your definition. Cut a tonneau in half and you have those backseats of the cars.

(oh and it is also a "360" - when a car has an accident and flips completely around...and when a plane does 360 degree flips sideways)