They are quite pricey at $229.95 for all three pieces.  I have not seen any
cars with these on them or had any experience with them.

http://www.karkraft.com/99-02_mustang.htm

What about having a body shop paint the area on the back of the car between
the taillights matt black?

I had that done as well as painting the rear wing matt black and it looks
sharp on a red car.  On a yellow car it should look even better.
It cost me about 500 to have those pieces painted as well as paint and
install a new hood.

Here is a link to the pics.

http://www.mustangforums.com/photosearch.asp?sortTypes=6&titlefilter=&memfilter=
rplourdejr&timefilter=&hitfilter=&ratefilter=&commentfilter=&page=1