Try this page and scroll down to the splash guards. The ones with the white outlined pony emblem on them are supposedly for both front and rear applications up through '07 Mustangs.
Splash The Guard!!!
With all my traveling, I have had to get tar streaks, gum and scum off my rear fenders. I will also be doing a fair amount of gravel and dirt road travels in the near future.... so.... I just got in a set of moulded, Ford splash guards for the rear. These are custom fit for the V6 and say "Mustang" in raised lettering. They fit perfectly and because they are moulded, they are solid and do not flap around. They also just cover the tires so you can barely notice that they are there. They do not look bad at all for that matter.

And they do reduce the crud from getting on the rear fender a great deal!