2020+ Super Duty Upfitter Aux Switch Output Wires — Exact Locations and How to Identify Them

2020+ Super Duty Upfitter Aux Switch Output Wires — Exact Locations and How to Identify Them

Quick Summary

On 2020+ Ford Super Duty trucks equipped with factory upfitter (aux) switches, Ford provides factory blunt-cut (sealed) wire leads for each aux switch circuit. The most common reason people struggle is because there are multiple “upfitter-related” wire bundles (outputs vs pass-through), and the outputs are often taped up tightly with a single harness label (not individually tagged wires).

This post explains the exact locations to check, how to tell outputs from pass-through wiring, and how to identify the correct circuit before you connect an accessory.

Important: This post applies to Super Duty. F-150 wiring layouts and ratings are different.

What You’re Actually Looking For (Two Different Things)

There are TWO different sets of wires people confuse:

  1. Aux switch OUTPUT wires (what you connect accessories to).  These are the factory blunt-cut/sealed leads associated with the upfitter/aux switch system.
  2. Pass-through wires (a routing option through the cab/firewall).  These are not the primary “outputs” people expect to find first. They’re simply a way to route wiring through the vehicle.

If you mix these up, you’ll waste time or connect to the wrong wires.

Location #1: Aux Switch Output Wires (Primary Connection Point)

Ford provides blunt-cut, sealed aux switch circuits that are intended for accessory installation.

Where to look:

  • Start near the upfitter/aux switch relay box / upfitter wiring bundle location on the passenger side area (vehicle-side wiring is typically accessed at/near the passenger footwell / kick panel area depending on build).
  • The wires are often taped into a harness and can be pulled down for access.
  • Expect blunt-cut sealed ends (not a plug you just snap into).

Labeling note (important):

  • The individual wires are typically NOT each labeled “AUX 1, AUX 2, …” on the insulation.
  • Instead, you’ll usually find a single label/tag on the bundled harness, and you identify each circuit by the factory color coding and verification testing.

How to identify the correct aux circuit:

  • Use the factory color coding for your year/build (owner documentation)
  • Confirm with a multimeter test while toggling each aux switch

Location #2: Pass-Through Wires (Commonly Confused With Outputs)

Ford also provides pass-through wires that run from the cab area through the firewall to the engine bay.

Where to look (interior side):

  • Under/near the passenger-side instrument panel or kick panel area (varies by model year/build).
  • These wires are often taped on a harness and may need to be pulled down to access.

Where to look (engine bay side):

  • The matching pass-through end is on the engine bay side in the corresponding harness area.

What these are best for:

  • Routing a trigger wire, switch signal, or low-current control wire cleanly through the firewall
  • They are not “the only place” to find upfitter outputs, and they should not be confused with the blunt-cut aux output leads.

How to Verify You’ve Got the Right Wires (Do This Every Time)

Do not rely on “it looks right.”

  1. Turn ignition to the state you expect (key-on vs off)
  2. Set multimeter to DC voltage
  3. Test the candidate wire while toggling AUX 1, then AUX 2, etc.
  4. Confirm which switch controls which wire
  5. Only then connect your accessory (or better: use the aux wire to trigger a relay)

This takes 3 minutes and prevents hours of troubleshooting.

Common Mistakes

  • Expecting individually labeled AUX 1–AUX 6 tags on each wire
  • Confusing pass-through wires with aux outputs
  • Grabbing a nearby bundle that is actually PTO/other circuits
  • Not verifying with a meter before connecting an accessory

OTC Perspective (No Hype, Just Practical)

Even if you never buy anything from OTC, follow this rule: identify the correct circuit using factory color coding and a multimeter before connecting accessories. Ford’s wiring is consistent enough to work with, but builds vary enough that assumptions will burn you.

Sources and References (for trust + images)

Ford Owner Manual — Locating the Auxiliary Switch Wiring (shows blunt-cut sealed wires and under-hood relay/fuse box context)
https://www.fordservicecontent.com/Ford_Content/vdirsnet/OwnerManual/Home/Content?ProcUid=G2449604&Uid=G2449627&buildtype=web&countryCode=USA&div=f&languageCode=en&moidRef=G2092022&userMarket=USA&vFilteringEnabled=False&variantid=9723

Ford-Trucks.com — 2024 Super Duty upfitter interior pass-through wires found (shows taped bundle and single harness tag reality)
https://www.ford-trucks.com/forums/1746924-2024-super-duty-upfitter-interior-pass-through-wires-found.html

FordTremor.com — Accessing upfitter and pass-through wiring (owner walkthroughs and access methods)
https://www.fordtremor.com/threads/six-easy-ways-to-reach-your-pass-thru-wires-for-up-fitters-and-profit.5383/

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