I thought the Chrysler 300C STR8 = Fast , Ambient posters from Brazil were made of plastic because I was thinking of the concept idea semi-transparent resin billboard for Mustang. (which is a portfolio piece not a real ad)

They're not plastic though, like the Volkswagen R32 ads, they're photos of the background, blurred.

Volkswagen R32 posters, released January 2008.
Agency : Ogilvy Cape Town, South Africa
Executive Creative Director: Gordon Ray / Chris Gotz
Creative Director: Gordon Ray / Chris Gotz
Art Director: Jennifer Macfarlane
Copywriter: Cuanan Cronwright

Big clue in how it was done, and that it was done, is in the extra photos submitted by Ogilvy Cape Town. The posters are photos of the background, blurred.

Credits for Chrysler 300C STR8 here. Brazil, May 2008.

Since the creation of these billboards involve "Copying" the background, I'm sure that I'm not the only one reminded of the copy concepts and the fab transparency ads for Amnesty International