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FastWire S Installation

Installing the FastWire S Wiring Harness

Tools and Materials Needed

  •  Zip ties and a side cutter or strong scissors
  • Sockets and/or ring spanners for the battery terminals
  • Screwdrivers
  • Drill and drill bits or stepper bit (for switch installation)


Safety Precautions

  • Disconnect the battery terminals from the battery before starting the installation process.
  • Avoid drilling into important vehicle components.


Important Note

The FastWire S Harness kit comes with piggyback adaptors for both HB3 and H4 halogen headlights which cover a vast collection of Japanese, European and American vehicles. Newer models may be fitted with Xenon or LED headlights which will require specialty adaptors not included in the kit. If in doubt, seek professional advice.

DO NOT attempt to splice into your headlight wiring loom unless you are certain that you know what you’re doing! Some vehicles (e.g., the Nissan Navara NP300) are very susceptible to headlight control module failure due to voltage spikes, wiring short circuits or incorrect wiring connections.

Before You Start

Install the light bar. The optimal wiring run will depend on the final position of your bar, so make sure that you’re happy with the position of the light and it’s mounts before installing the harness.

Wiring harness schematic diagram
FastWire S Wiring Harness

Step 1 – Install the Wiring Harness

Lay out the wiring harness on a flat surface and identify each component. Now lay it out roughly in the engine bay so you can visualise where you will run the wires and components.

Choose your preferred location for the relay. It needs to be close to the battery to keep the high current power and earth wire runs as short as possible but should not interfere with other serviceable vehicle components.

Identify where you will run the switch wires through the vehicle’s firewall. There is usually an existing rubber grommet that can be used for this purpose.

Now test run the harness in the vehicle, ensuring all wire lengths are adequate. Avoid sharp edges, suspension or exhaust components or other items that could melt or cut the wire insulation. If you need to lengthen or shorten wiring runs, make sure to join the wires using soldered and shrink tube protected connections or waterproof Deutsch plugs.

When you’re satisfied with the location of all the components, go ahead and secure the relay assembly using an existing bolt or screw in the engine bay. Alternatively, carefully drill a new hole and screw down the relay bracket.

Run the wires in the chosen locations and secure them using cable ties. DO NOT connect the power terminals to the battery or power bus bar at this point.

Wiring harness high beam adapters


Locate the vehicle’s headlight wiring harness where it connects to the most convenient headlight high beam bulb assembly and disconnect it. Connect the headlight adaptor to the vehicle’s high beam connection and re-connect the headlight harness to the other end of the ‘piggyback’ adaptor. Now connect the FastWire S headlight wires from the relay to the matching white ‘T’ connector on the headlight adaptor.

NOTE: If you’re using a vehicle specific headlight adaptor other than the HB3 or H4 adaptors supplied with the harness, it may not connect to the high beam bulb connector but directly to the headlight power harness instead. This is the likely scenario for HID and LED headlight assemblies which have a single harness connection.

Light bar switch

STEP 3: Install the Switch

Decide where you would like to install the switch on the vehicle’s interior. This could be on the dashboard, centre console, or another location that is easily accessible to the driver. You can use an alternate switch if you wish (e.g. Carling type or vehicle specific design)

Use a drill to create a hole for the switch. A stepper bit is useful for this but be careful to protect exposed surfaces of the interior from damage.

Disconnect the supplied switch from the harness, noting the order of the wires. Tape up the wire ends and feed the wire through the firewall to the switch location. Use a mousing wire and some silicone spray to make this process easier.

Insert the switch into the hole, secure it in place and then connect the switch to the black, blue and white wires in the wiring harness.

Wiring battery connection

STEP 4: Connect the Relay

Remove the fuse from the wiring harness fuse holder.

Connect the red wire from the wiring harness to the positive terminal of the battery or the positive side of a battery bus bar (if fitted).

Connect the black wire from the wiring harness to the negative battery terminal or a ground point on the vehicle. We recommend earthing at the battery terminal.

Light bar test

STEP 5: Connect, Check and Test

Connect the Deutsch connector on the wiring harness to the corresponding connector on the light bar if you haven’t already done so.

Re-connect the battery terminals if they are still disconnected, positive terminal first.

Check your installation. Make sure all connections are firm and that the wiring is not pinched, cut or stressed.

Insert the blade fuse into the harnesses’ fuse holder and turn on your vehicle’s headlights.

Turn on the switch you installed. With the headlights on low beam, the light bar should not operate and the red light on the switch will be off.

Switch your headlights to high beam. The light bar should now operate and the red indicator light on the switch will illuminate, indicating the light bar is on.

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