1. Check the Wiring Harness and Connections:
* Visual Inspection: Carefully examine the wiring harness leading from the switch to the motor and regulator. Look for any broken, frayed, or corroded wires. Pay close attention to the connectors – ensure they're securely plugged in and the pins aren't bent or damaged. Wiggling the connectors while testing the window can sometimes reveal intermittent problems.
* Continuity Test (Advanced): Using a multimeter, check for continuity between the switch and the motor. You'll need a wiring diagram for your specific Mustang to identify the correct wires. This will tell you if there's a break in the circuit. Caution: Disconnect the battery negative terminal before performing any electrical tests.
* Common Break Points: Look particularly at the wiring near the door jam – this area flexes frequently and is prone to wire damage. Also check the wiring where it enters the door itself, as it can be pinched or rubbed against the door frame.
2. Inspect the Window Regulator:
* Visual Inspection: The regulator mechanism itself can be faulty even if the motor works. Look for broken or bent parts, especially the plastic clips and guides that hold the window in place. You might need to remove the door panel to inspect the regulator thoroughly.
* Testing (Advanced): With the door panel removed, you can try applying 12V directly to the motor leads (with the correct polarity - consult a wiring diagram!). If the motor works, the problem is likely the regulator or its connection to the motor. If the motor *doesn't* work, even with direct power, then your initial diagnosis of a good motor is incorrect.
3. Check the Ground:
* Ground Connection: A poor ground connection can prevent the window from working. Locate the ground wire for the window motor and regulator (again, a wiring diagram is crucial). Clean the ground point with a wire brush and ensure a solid connection.
4. Consider the Master Switch (Driver's Side):
* Passenger Window Circuit on Master Switch: Although you've tested the passenger switch, the problem might reside in the *driver's side* master switch. The passenger window circuit within the master switch could be faulty, preventing the passenger window from operating even if the passenger switch itself is fine. Test the passenger window using only the driver's side switch.
5. Fuse Check:
* Check Related Fuses: Consult your owner's manual to identify any fuses related to power windows. A blown fuse is a simple fix.
Important Considerations:
* Door Panel Removal: To access the regulator and wiring effectively, you'll likely need to remove the door panel. There are many online tutorials showing how to do this for a 1999 Mustang.
* Wiring Diagram: A wiring diagram specific to your 1999 Mustang model is essential for proper testing and repair. You can usually find these online through forums dedicated to Ford Mustangs or via a repair manual.
* Safety: Disconnect the negative battery terminal before working on any electrical components.
If you've gone through these steps and the problem persists, it's best to seek professional help from an automotive electrician or mechanic. They have the tools and expertise to diagnose more complex electrical issues.
