Get the special tool to remove the trim from around the windshield and rear glass
LINKY
You can try your local NAPA store or call LRS, they might have them also.
If you try to pry the trim off with a screwdriver, you risk damaging the trim pieces, or breaking the clips.
To use the tool, place the jaws of it flat on the glass. Orient the tool so that the point of the little hook on the end of one arm goes under the trim first, then slide it under until you feel the point of the hook hit the back of the trim. Holding the tool in that position, slide the tool along the glass (in the direction of the handle), until you fee the hook catch a clip. Then rotate the handle away from the trim (in plane with the glass). The hook will pry up the clip, releasing the trim. Remember, you are never prying up on the trim, just releasing it from the clips -- the trip should pop up on its own.
It's very hard to picture how this works, and I haven't found a diagram that shows it (not even in the Ford Mustang Service Manual), and the tool doesn't even come with instructions. But one you've sucessfully tried it, you'll be amazed at how simple it actually is.