Try the clutch switch first...you can jump it with alligator clips before you replace it, or try putting an ohm meter across the contacts to see if the swicth is working.

 

I don't know if your Mustang has it, but mine came with a dealer installed ignition cut off switch chip that plugs into a socket just under the dash below the steering wheel. It's easy to knock out unkowingly with your knee. If it isn't in the socket the car will not turn over at all. It can sometimes be jarred loose enough but remian in the socket to keep it fom turning the ignition on. Hopefully you can find out what the problem is.

 

Check the cables that run to the clutch sensor, they may have a short.

 

Well we finally figured out what was wrong with car. It turned out to be the clutch sensor. She took it to a repair shop the other day and they told her it was the alarm that was causing the problem and to find someone who deals with that alarm. We found a place that would check it out for us and see if that was the problem. Well my dad went an checked it out and fixed it himself.

What had happened is that it would start some of the time and other times it would not. Evidently one of the wires had become loose and caused it to not work all of the time. So she took it to my uncle to have him look at it. My uncle can fix any diesel engine around since he owns his own pulpwood company, but he cannot fix cars. So he pulls the wires off of the clutch sensor and leaves them off. I tried to tell my dad and then he starts yelling at me saying I didn't know what I was talking about. I told him about what I had found here. Well this morning he puts the wires back on and puts them back on tight. Started up first try.

Thanks guys for all your inputs and now I know where to go if I need information like this again.