Please excuse my reply if it doesnt make much sense, as I dont know much about the stangs !
Anyway, most probable cause would be the Power window switch.
Its common for the contacts to wear out after some period of time, or repeated use.
The only I can suggest at the moment, is to swap switches (If possible) with the passenger side one, and see if the problem is transfered to the other window.
If that fails, check to see if the switch is getting power.
finally, for a last test, U could try and bypass the switch all together, and connect one wire from the power window motor to ground, and the other to the battery, and see if you get any movement.
As I said above though, most likely cause is the switch becoming a bit old and worn out.
Hope this helps,