this car just needs help

OK the first thing you will need are good wiring diagrams.

I recommend the Haynes Service Manual for the Ford Mustang 1979-1993 and Mercury Capri 1979-1986. The stock number is 36050.

Familiarize yourself with the wiring diagrams in the back of the book and then try to find a friend with a mustang that has the same power accessories or find a mustang with similar equipment in the junkyard. The junkyard is a place of solutions on so many levels. Don't just go there to buy parts but go there to study cars that are often partly dismantled and allow you to easily study areas not easily accessible on your own car.

As for the passenger-side window, the switch may be bad. Its both cheap and easy to install a replacement switch. If the motor can lower the window, it most likely works. Does the switch on the drivers side move the passenger window up and down or just down too???

Open up the passenger door and use the opportunity to spray some white lithium grease on all the moving parts (everything including door handle rods and lock assembly). Put a voltmeter on the plug to the power window motor and see if you are getting 12 volts when the switch is used in the up and down position. Have a friend use the switch in the driver's console as well. Check for any breaks in the wires, or exposed insulation, and make sure the wires passing through the door grommet are not damaged. If you are getting volts from both switches, to the plug that goes into the motor, in the up and down positions, then the motor is at fault otherwise the switch(es) are bad.