Take the inside quarter trim out of the back seat and you will be able to access the fasteners that hold the side scoops on. Be very careful when working with the cheap crappy easy to break plastic interior trim on your Mustang. Sometimes it takes a couple tries to get it back on properly...
Also, some people say that you can just grab the side scoop and just pull them off...
you can get them off from the outside if you are careful, there are 5 or so little black pins around the scoop. you can use a flat thin screwdriver to put pressure on the little latch part and they will pop out. just be careful and it will be a piece of cake. the scoop will flex away from the car a little so you can get at them. be careful = easy