I've seen a few posts on people asking how to do different things as far as removing the door panel and it's pieces and I know a lot of people buy new speakers so I figured I'd make up a little walk-through with pics and what-not.
WARNING: I'm extremely bad at photoshop so I used my most basic skills on this... flame away.
The Mustang actually has one of the easiest door to take off, but one of the hardest to put back on. It's quite simple once you get the hang of it.
The first thing you'll need to do in order to get the door panel removed is to take out the bezel in the middle of the door. You'll need to pop out the top part of the bezel and lift up slightly, then with both hands, grab the bezel and tug firmly on it in a straight upward direction in order to pop the clip out of it's socket. The clip is located where the bottom red circle is there so you want to place your hands around that and pull straight out so you don't snap this. I remind you that anything you break on this bezel ruins the entire bezel because it won't fit right and you have to buy a new one (23 bucks from Randy
had to get one myself). Also, be careful not to pull the bezel too far out because your wiring is still all hooked up to your switches.
When you turn the bezel around and look at the back of the switches you have 2 screws and a clip. Just pull the clip off and unscrew the two screws to release the switches from the bezel. I suggest pulling gently on the wires to remove the switches from the switch housing because if you pull the switch housing from the bezel, you must buy a new bezel because your switches will be sitting inside your door.
Next thing is to remove this plastic plug from the door panel. It's located near the bottom edge of the door where the door hinges are. It's a bitch to pull out. You can use as much force as nessecary to pull this sucker out, if it breaks you can buy bags of these for a couple bucks at your local auto parts store. Most likely it won't break and it'll just bend the frindges in the opposite direction which will work out nicely because they'll need to be facing that way when you put it back in the door.