2003 Mustang v6 Convertible (window issue)
 

Passenger window won’t go up. Disconnected harness – tested voltage from harness (Door switch) - pressed UP read 12v (yel/red), 0 on down (red/yel) – pressed DOWN read 12v down and 0 on up. Connected switch harness to regulator and tested switch again. This time pressing up, harness read 12v up and 12v down (issue). Pressing down voltage ok both wires. Took a stand alone battery and was able to raise the window by connecting regulator harness directly to the battery, regulator harness was disconnected from switch harness. Cannot understand why I get 12v from both up and down on switch only when harness is connected. I replaced both master and passenger switches with no results. Anyone?
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Welcome to the forums! :wave:

Ok, since you've replaced both switches, the motor is verified working, so... yeah, the wiring is the problem. You'll have to trace each pin out and verify there's no shorts.

https://cardiagn.com/2003-ford-mustang-workshop-manual/

Download the wiring diagram at the bottom and print it out and go to town. The system is not complicated with modules or such. You've likely got a wire or two that's chafed or corroded. You can probably find a diagnostic in there too that'll point you into properly testing things, but you're gonna want to test for both continuity end to end as well as proper voltage and/or cross shorts.

Also notice how the passenger switch is completely dependent on the driver's switch. Perhaps a test is to somehow wire up 12V directly to the passenger switch and separate it from the driver's side so it can be independently tested and verify the problem is between the driver's door and the passenger door. In any event, tedious testing is in order.

Not the answer you're lookin' for I imagine. I hope it helps anyway, and good luck! :nice:

Thank you for the great feedback. Found no continuity on the tan/lt blu wire running from driver door switch to passenger switch. All others ok. Finding that break my prove endless so I think it best if I run a new wire. Either way, should be fun. Thanks again.

Ran new wire, window back in operation.