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HomeTips & GuidesAuto Dimming Mirror Wire Harness: How It Works and the Installation Process

Auto Dimming Mirror Wire Harness: How It Works and the Installation Process

Auto-dimming mirrors are unique rear-view mirrors that can dim the light reflecting from their surface.  The underlying mechanism that enables the dimming function is a set of electrical components that require an auto-dimming mirror wire harness for connectivity.

This feature is beneficial, especially for night driving, because it eliminates eye strain caused by blinding light reflections from the car behind. 

Let’s look at everything you need about this auto-dimming mirror and its wire harness.

Table of Contents

How Auto-Dimming Mirrors Work

Before entering its wire harness, let’s examine how these electrochromic devices work. The power mirrors contain three electronic components.

  • Ambient Sensor: The rear-view mirror faces the back side of the car. Its ambient sensor faces the vehicle’s front, meaning it is on the rear side of this mirror in the housing. This photosensor detects ambient light from the front of the vehicle.
  • Glare Sensor: A glare sensor faces the rear side of the vehicle to detect the light intensity from the cars behind.
  • Control Unit: This component is the brain of the auto-dimming function. It takes the data from both sensors to decide when to dim the mirror.
Photoresistors for measuring light intensity

Photoresistors for measuring light intensity

If the amount of light detected on the glare sensor is more than the intensity detected on the ambient sensor, the control unit gives the signal to dim the mirror.

Dimming Function

The mirror is not your regular type. To enable dimming, it features mirror and glass layers that sandwich five inner layers.

With the mirror on the vehicle’s front and the plane glass facing the rear, here’s the layer arrangement.

  • Mirror
  • Conductive layer
  • Electrochromic layer
  • Electrolyte
  • Ion storage
  • Conductive layer
  • Glass

When the control unit decides to activate the auto-dim feature, it sends an electric voltage to the two conductive layers. 

This voltage causes ions to move from the ion storage layer to the electrochromic layer via the electrolyte.

Ions in the electrochromic layer darken the gel due to oxidation, reducing the light reflected from the mirror behind it. This principle is known as electrochromism.

The electrochromic gel changes color depending on the electrical charge applied to it. The process is reversible to allow maximum light reflection when the glare reduces from the rear.

A dimming rear-view mirror

A dimming rear-view mirror

Benefits of Auto Dimming Mirrors

  • Enhances the night driving experience
  • Improves safety
  • Reduces driver fatigue caused by eye strain
  • Improves visibility

What Is an Auto-Dimming Mirror Wire Harness?

Since the auto-dimming mirror is an electrical device, it needs power from the vehicle’s battery or power system. Therefore, it must have an auto-dimming mirror wire harness to make it operational.

This harness has the following components.

  • Electrical Connections: The harness links the auto-dimming mirror to the car’s electrical system using wires and connectors. It features a power supply line that brings in electrical power to enable the control unit to apply voltage to the conductive layers when needed, depending on the sensor inputs.
  • Sensor Integration: This harness connects the two photo sensors to the control module to channel the data signals.
  • Rugged Insulation: This harness must be durable enough to withstand extreme temperatures, vibrations, and other conditions in the vehicle. The insulation should provide this protection.
A woman adjusting the rear-view mirror in her vehicle

A woman adjusting the rear-view mirror in her vehicle

How To Install an Auto-Dimming Mirror Wire Harness

This installation process can vary depending on your vehicle’s make and model. But you’ll need these items and follow the general steps below.

  • Vehicle wiring diagram
  • Auto-dimming rear-view mirror
  • Dimming mirror harness (if not included with the mirror)
  • Plastic trim removal tools
  • Screwdriver/socket set
  • Heat-shrink tubing or electrical tape
  • Heat gun
  • Wire connectors
  • Crimping tool
  • Wire stripper

Step 1: Disconnect the Battery

Park your car in a well-lit area, open the hood, and disconnect the negative battery terminal from the lug.

Step 2: Remove the Stock Mirror

Use a socket or screwdriver to remove the factory mirror in your vehicle, and take extra caution if removing it from the windshield mounting. Unmount the mirror carefully from its bracket because you can reuse it later or sell it second-hand.

A windshield-mounted rear-view mirror

A windshield-mounted rear-view mirror

Step 3: Prepare the Auto Dimming Mirror for Installation

Most auto-dimming mirrors come with wire harnesses pre-installed. If your accessory mirror does not have it, prepare it by crimping wires to the pigtails hanging on the device. Connect the wires accurately and cover them using heat shrink tubing.

For easy installation, ensure the new auto-dimming mirror is compatible with the mounting bracket for the old mirror. If not, you’ll have to customize this part. Mount the rearview mirror replacement, then proceed to wire routing.

Step 4: Route the Auto Dimming Mirror Wire Harness

The closest place to connect the auto-dimming wire harness is the courtesy light.

But if your vehicle, for any reason, does not have this feature, you’ll have to go through the headliner and then down to the dashboard via one of the A-pillars.

You’ll need a plastic trim remover for this task.

Step 5: Connect the Wiring Harness

Use the vehicle’s wiring diagram to determine the polarity, function, and voltage of the nearest wires you can tap into.

Let’s consider the courtesy light wiring connection. You don’t have to remove any plastic trim for this installation.

A rear-view mirror installation near the courtesy lights

A rear-view mirror installation near the courtesy lights

Undo this roof light, then check the wiring connectors used on it. Most have spade pin connectors that make it possible to use piggyback mirror connectors to draw the power.

Depending on your situation, you might have to cut the wire for crimping or soldering to tap into the electric current.

Insulate the connections using electrical tape or heat-shrink tubing, then double-check the installation.

Step 6: Test the Dimming Feature

Reconnect the negative battery terminal to test the mirror’s dimming feature. It should switch automatically after pressing the button because the courtesy light wiring has an always-on 12V power supply line.

But if you connect it to an ignition-switched power line in the dashboard, insert the key and turn on the vehicle to activate the mirror.

A rear-view mirror with an adaptive dimming system

A rear-view mirror with an adaptive dimming system

Check if the mirror responds to changing glare conditions. Try covering the car-front-facing ambient sensor with your hand and observe the dimming. If it functions as required, proceed to the next step.

Step 7: Secure the Wiring Harness

If the wires go down the A-pillar, tuck them neatly on the structure and secure the harness using cable ties. Next, reattach the plastic trims.

This step is not necessary if you tap current from the courtesy light. But you’ll have to re-mount this light and secure it firmly.

 Confirm if this component also works, then test the dimming mirror again when the installation is complete.

Wrap Up

An auto-dimming mirror is a nice upgrade to your vehicle because it will make your driving experience safer and more comfortable in the dark. 

Wiring this rearview mirror is one of the simplest electrical tasks in vehicles, and the guide above should help you complete the setup.

That marks the end of this article. Check out this power window wiring guide for another DIY car wiring project. 

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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