Starters are a funny anomoly. They work just about opposite of any other kind of electronic device. The faster it turns, the less current it uses. Double the voltage and you quadruple the power. Freeze it up and you can turn a 4 guage copper wire into a giant matchstick. If you slowly lower the voltage to a resistor, or lamp, or heater, that device just gets cooler or dimmer. But lower the voltage to a starter and you had better have fire insurance.
If you have a high compression motor (probably not on a mustang) starters don't last long. But put that same starter on a low compression engine and it might last 15 years. If you have a loose or dirty connection in your high current path (the big red and black wires) you will be kicking yourself in the head troubleshooting the problem (that is unless you encountered that problem before and learned).
In my lifetime I have rebuilt over thirty thousand starters, thirty thousand alternators (and generators), thousands of selonoids, regulators, and devices I never even knew existed until someone brought them to me to be repaired.
Ninety percent of the causes of bad or burnt starters (90%!) are bad batteries. If I had to guess without any information what caused a starter to go bad, I would have to say the battery. The next cause of starting problems is dirty terminals. The third cause of starting problems is a faulty relay (if you knew the horror stories about relays, you would change yours every two years without question), and the final (itsy bitsy tiny percent) are things like a neutral safety switch or ignition switch.
One time a woman came to my sisters complaining about her gorgeous 76 Electra Limited and all its problems. She was even considering selling it. She had replaced the starter, the alternator, the ignition coil, the ignition module, and so on. I asked her what it would be worth to her to solve the problem. She said "$50.00 dollars."
I told her to change the battery. She yelled at me for what seemed like an hour refusing to believe that no mechanic has yet to tell her that and that I was way out in left field. As, at the time, I was heading into the military and had a long drive still to go I just shrugged it off and started my journey into my military life.
At basic training, I received a call from my sister . She said that her friend wanted to know where to send the 50 bucks.
I guess what I am trying to say in my usually NOT short fashion is that before you change a starter, test and troubleshoot all you can, and make sure your battery is good.
Doc