There are many unused fuses populated in my 2010 Ford Fusion's interior fuse box. I've tested with a voltmeter and the fuse I have in mind turns on and off with the ignition, which is what I want. Now, I need to find the output wire hooked up to this specific unused fuse.

How can I figure out which wire I want to tap into without completely destroying my wiring harness? (If it matters, it's fuse #34.)

I know I can also accomplish what I want via a fuse tap also, I just thought that if there is a wiring harness existing, it would be awesome to just use that. And a tap would make it hard to fit the cover back on the top of the fuse box.

2 Answers 2


Easiest way to do what your asking is use a digital multi-meter in continuity mode but first you want see which side of the fuse is connected to the supply so set your meter to DC and remove the fuse, turn your ignition on and check voltage at the fuse terminals. Which ever side of the terminals read 12V is your supply, next for simplicity, remove the SJB (aka interior fuse box, BCM) and set your dmm to continuity mode. Place one probe in the non-supply terminal of the fuse position you want and then use the other probe to check the connector pins until you find one or more that beeps. Take note of which pin(s) beep and compare them to the harness inside the car. The cavity should be empty if not in use. You could then buy a terminal from dealer and run your own wire through harness.


A quick way would be to install an inline fuseholder and connect it to the hot side of an existing "Hot on run" fuse.

  • Best solution, that way you do know power the unknown circuit.
    – Moab
    Commented Nov 3, 2019 at 19:37

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