Norm:

not exactly sure what kind of "clear plastic things" you refer to, but
my experieces:

 several cars, tried a molded lexan or plexiglass wind-dam sort of
thing that was supposed to divert dirt and stones from the windshield
(and also covered several inches of the front of the hood).  yeah, they
diverted a negligible amount of debris from the windshield.  they also
made the wipers float at speed .....then, I found out that you cure the
floating wipers with heavy duty springs to hold them down.......then I
found out that wears out the wiper motor.  enough of that.

,  for my '06 Mustang:  got the "factory approved" bra set-up......the
most miserably designed, hard to install monstrosity I've ever
encountered. Took over an hour to get it on BUT.....a very tailored
installation and looks pretty good.  after a year and a half, it hasn't
abraided the paint underneath.

that left the headlights which look very vulnerable.....and expensive.
when I got the car I went online and ordered catalogs from every vendor
who claimed to have '05 and up accessories.  got a bunch of catalogs
that didn't have anything for the new model.  so I hit eBay......several
vendors selling molded plastic headlight covers.....but after 2 months
of trying, nobuddy had clear ones, just blue,smoked, bronze,red,etc.

so finally, just bought a set from another vendor on eBay of
self-adhesive mylar film covers.  at $14.95 they were horrbly overpriced
for what they are, but they did arrive pre-cut and you just peel off the
backing and stick them on.....can't even tell they're on there, and I
trust will keep the lenses from clouding from abrasions.

Hope this helps.