How to Wire a Reverse Camera to Your Head Unit (With a Manual Switch)

I installed an aftermarket reverse camera on my Land Cruiser Prado (120 series), connected to the head unit's camera input. I wanted two things from it:

1. Automatic: the camera comes on and the head unit shows it whenever I select reverse.
2. Manual: I can switch the camera on myself at any time, not just in reverse.

The camera itself is the easy part. The wiring is where people (including me) get caught out, so this post is mostly about that. The principle is the same for almost any car and head unit.

The parts and the wires that matter

At the head unit you need two connections:

• Video in (RCA): usually a yellow RCA socket on the back of the head unit, often labelled "camera" or "rear cam".
• Reverse trigger input: a wire that tells the head unit to switch to the camera picture when it sees +12V. It's an input: you have to feed it 12V when reversing. Your head unit's wiring diagram will show which wire it is.

At the camera there are usually three things:

• The video lead, which runs to the head unit through a long RCA extension cable.
• Red: +12V power.
• Black: ground.

Many cameras also have small loop wires you can cut to flip the image or remove the parking guidelines. Check the camera's instructions before you cut anything.

At the back of the car, the reverse light wire is live (+12V) only when the car is in reverse. That makes it the perfect power source for the camera and the trigger.

The wiring diagram

How it works:

This is the wiring I've run on my Prado for years:

• In reverse: the reverse light power powers the camera, and through the diode it also feeds the head unit's reverse signal line, so the head unit switches to the camera.
• With the manual switch on: the head unit's +12V line goes through the switch to the camera and to the reverse signal line, so I can bring the camera up at any time.
• The diode sits between the reverse light power and the reverse signal line, so switching the camera on manually doesn't power the reverse light through that line.

The diode is a 1N4007: a few cents, rated at 1A, which is plenty here. The stripe (cathode) faces away from the reverse light, towards the reverse signal line. It's a small part, but it's important: solder it in.

Where to get the switched 12V

Use a source that is only live with the ignition (ACC) on, so the camera can't flatten the battery if you leave the switch on. Two common options:

• the head unit's remote / amp turn-on output (often blue/white), which is live whenever the head unit is on. Cameras typically draw well under its rated current, but check both.
• an add-a-circuit fuse tap on an ACC circuit in the fuse box.

Fuses

Put a small inline fuse (1–2A) on each feed you create, close to where you tap in. Without a fuse, a pinched wire behind a trim panel can blow the car's own fuse for that circuit, or worse.

I learned this the hard way. During my install, the indicators and the reverse lights stopped working: I'd blown one of the car's fuses. Replacing the fuse fixed it. Keep spare fuses of the right ratings on hand, and disconnect the battery's negative terminal before you cut into anything.

Step by step

1. Disconnect the battery's negative terminal.
2. Find the reverse light wire. Remove the trim behind the tail light, switch the ignition on (engine off), select reverse with your foot on the brake, and use a multimeter or test light to find the wire that shows 12V only in reverse. Then disconnect the battery again.
3. Mount the camera. Centred is easiest because the picture and the guidelines are symmetrical, but a little off-centre works fine. Many cars have a number-plate light or a blanked hole you can use. If you need to drill, check what's behind the panel first, and seal the cable entry with a rubber grommet or silicone so water can't get in.
4. Run the cables forward. Route the RCA cable and a power wire from the back of the car to the dash, under the door sill trims or through the headlining. In wagons and SUVs the trickiest part is getting from the tailgate into the body; use the existing rubber boot that the tailgate wiring passes through.
5. Make the connections as in the diagram: camera red to the reverse light power, the diode from the reverse light power to the reverse signal line, the switch between the head unit's +12V line and the camera/reverse signal line, the RCA cable to the head unit, and the camera's black wire to a clean, bare-metal ground point.
6. Solder and insulate. Solder the taps and the diode, cover them with heat shrink, and tape and tie the wires so nothing rubs on sharp metal.
7. Mount the switch somewhere you can reach from the driver's seat.
8. Reconnect the battery and test, before you refit any trim (see the checklist below).

For tapping into the reverse light wire, Super Cheap Auto's How to Install a Reverse Camera video is the clearest I found. The tapping part starts at 1:10.

Test before you put the trim back

• Ignition on, reverse selected: the camera picture appears on the head unit.
• Ignition on, manual switch on, car in park or neutral: the camera picture appears, and the reversing lights stay off.
• Indicators, brake lights and reversing lights all still work normally.
• Ignition off, switch on: nothing is powered.

Troubleshooting

• No picture in reverse, but the head unit switches to the camera screen: the trigger is working, so it's power or video. Check for 12V at the camera's red wire in reverse, then check the ground and the RCA connections.
• Head unit doesn't switch to the camera in reverse: check the reverse trigger wire for 12V when in reverse, and that the head unit's camera input is enabled in its settings.
• Works in reverse but not with the switch: check the +12V line from the head unit and the switch connections.
• Reversing lights come on when the switch is on: check the diode is in place and the right way round.
• Indicators or reverse lights stopped working: you've blown a car fuse while tapping in. That's what happened to me.
• Picture is mirrored, or the guidelines are wrong: look for the camera's loop wires or the head unit's camera settings.
• Picture flickers or goes blank in reverse on a newer car: some cars with LED tail lights or bulb monitoring pulse the reverse light circuit. Power the camera from a switched 12V source instead and use the reverse wire only as a trigger, or use a relay or a trigger module made for your car.

Tools and materials

• Multimeter or 12V test light
• 1N4007 diode
• Inline fuse holders with 1–2A fuses, plus spares of your car's fuse ratings
• Automotive wire (4–5 m for most cars), solder, heat shrink and electrical tape
• Soldering iron, wire strippers, crimper and crimp terminals
• A small panel-mount switch
• Trim removal tools; a drill and grommet if you need a new hole

Once you understand the diagram, the rest is careful routing and tidy connections. On my Prado everything has worked well since.

Credits

Photo by Evaldas Grižas on Unsplash