When you are done you could have something like this https://www.youtube.com/watch?v=owsrApBPuQM . (This is using the torque app, an hdmi to rca converter (powered via a usb car charger), and an android phone).
I, like many car enthusiasts, had an OEM headunit that is a touchscreen with a fairly sizable display that lacks an aux input functionality. My car specifically is a 2013 Subaru BRZ (aka the FRS, aka the GT86), but I will attempt to write this up in a generic way to assist those with cars from any manufacturer that gives you rear view camera functionality, but no aux input functionality.
First off, a few caveats, reminders, etc.:
-Don't use this to play distracting videos while you are driving. In many countries (and states in the US) this is illegal and carries stiff penalties. It is also dangerous. I use my input to display a mirror of my android phone running the torque applications which gives me OBD II gauges (http://torque-bhp.com/).
-Some manufacturers leave annoying guidelines and warnings on the screen that you cannot turn off. Using my method these will probably remain (unless their is a "hack" for your particular headunit/vehicle or it is something you can disable in settings (hidden or otherwise).
-This is not a tutorial on how to have both a backup camera and a reverse input, but the circuit to add the reverse input shouldn't be too difficult.
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Step 1: A List of Materials
1. A headunit that has rear camera input, but no aux input. It should accept generic cameras (RCA type signals) and not odd proprietary signals.
2. An RCA cable
3. A video input source
4. (Optional) A converter that does HDMI to RCA conversion. (I used a cheap generic one from amazon. It lasted a few months before it stopped working).
5. Wire crimpers
6. soldering iron and solder
7. heat shrink and/or electricians tape
8. wire strippers
9. and SPST switch of your choice
10. Documentation on how to safely remove the headunit in your car. Check with crutchfield and especially with the forums that enthusiasts of your particular make/model frequent (the forums really are a very valuable resource.)
11. Documentation of the pinout of your particular headunit (also probably available via your make/model specific car forum)
12. Information about the reverse signal. Is it 5v+ or GND to signal that the car is in reverse?
13. Female molex style pins (I usually salvage these from fan connectors in cold computers)
Step 2: Remove Your Headunit
It might be a good idea to disconnect the battery of your car for your safety and the safety of the electronics in your car before starting (disconnect the Negative terminal. It is the black one).
Remove the headunit completely and put it in a safe place.
I recommend using painters tape and rags to cover sharp edges and the screen. This will protect both the screen of your headunit and the interior trim of your car.
Step 3: Create Your RCA Cable
Take the RCA cable (I used a male to male one, which worked with my video source) and strip it with the wire strippers. It will have a signal wire in the middle inside some insulation and a layer of meshy stuff which is the ground shielding outside of that (see the picture).
The shielding will go to a ground pin on the headunit and the innermost wire will go to the rear camera video input pin on the headunit. I scavenge my female pins (which fit nicely over the male pins on the back of the headunit) from fan and other connectors on old computers. Solder these up and insulate them properly using electrical tape or heat shrink tubing.
Step 4: Test the RCA Input Connection
I always like to test one thing at a time. So, before you go and mess with the reverse wires and such, make sure that you can get video in through the wire you created on the pin (from the documentation you found)
I usually prop up the headunit, plug things in and use a known good video source (like a DVD player) and put the car in reverse to see if the video signal shows up on the screen.
Troubleshooting this step:
-Video looks grainy or otherwise crummy. Try making your cable again. Test the continuity on your cable, etc. Make sure everything is well insulated.
-No signal, nothing changes when you put it into reverse. Check to make sure the reverse camera option doesn't have to be enabled in a settings menu (which is possibly hidden).
Step 5: Make Your Switch Wire
This step has no pictures as it is highly dependent on your specific car. Usually what happens is that when you put the car in reverse a circuit is completed and the headunit sees this on the reverse pin (usually labeled "REV") and shows the input from the pin we used in the previous step.
So, instead of using the gear lever as the switch to tell the headunit you are in reverse you are going to use an SPST switch. There are two ways this could work. 5V -> SPST switch -> REV pin OR GND -> SPST switch -> REV. Here I must stress you READ THE DOCS for your specific car and headunit. Overcurrent protection circuits may/may not exist and hooking things up wrong could mess up the headunit.
I use the same technique for the pins on this wire that I used on the previous steps.
Step 6: Test Everything...before Putting the Dash Back Together
Don't be in a rush to be done. Test everything before you slide the headunit in, bolt it down and re-assemble the interior of your car.