Don't use car bras! For the love of god, why would you want to subject a car to a torture like that?
I mean, not only is it redundant ("hey, let's protect the front of my car so that it looks nice by covering it with a big, black, ugly piece of cloth, or a stupid, easily-noticable clear plastic thing!"), it can easily be counterproductive...The previous owner of my car kept a bra on it, ruining the paint underneath and leaving me with a bubbled, peeling area along the front edge of my hood.