Bricked my dash cam after connecting multiple cameras

Hi all, having a problem that I would love some advice on!

So first, some background:
My car has a head unit with an AHD backup camera. I recently purchased a dash cam which had an input for a rear camera. I decided rather than stick two cameras on the back of my car, which would be ugly, I’d connect the camera to both the head unit and the backup camera, so it could be used for both. And while I was at it, I bought a second identical AHD, put it in the interior, and connected it to the dash cam as well (controlled by a STDP switch to choose between it and the exterior camera). Both cameras are powered by the head unit, but the interior camera data line connects to the dash cam, and the exterior camera connects to both (see attached wiring diagram, which should make the setup much clearer).

I wired everything up and it worked perfectly! I was absolutely thrilled! I tested it for a bit then unplugged everything to clean up the wires tucked behind the dash. After redoing the wiring behind the dash everything still worked, so I gave myself a pat on the back and called it a day. Unfortunately I came back a few days later and found my dash cam completely dead. Nothing I did would get it to respond including resetting it, powering it via usb, etc. It was bricked.

So maybe that’s a coincidence, but it seems pretty likely I killed it with my shenanigans, and I’m not sure what I did. Maybe there was a vpp mismatch? If anyone could give me some suggestions for next steps before I plug a replacement dash cam in and potentially fry it, you’d be my savior. Thanks for any advice in advance!

Getting a DMM(Digital Multi-Meter) and measuring the voltage output from the Head Unit should tell you what’s going on. There should have been a voltage spec for the interior camera you’d purchased or something listed on the power supply which came with OR the wiring diagram.

Also, you might look for a better video signal switch than a simple SPDT switch.
maybe something like: 2 to 1 Channel RCA Video Intelligent Switcher for Car | eBay


okay first, thank you for your help, you’re absolutely awesome!

Unfortunately neither the head unit (which provided the cameras) or the dash cam came with any voltage specs or wiring diagram. I’ve tried reaching out to the manufacturers so I’m hopeful I’ll get them. I do have a multimeter though, so I’ll absolutely follow your suggestion and get some measurements.

For the ebay listing you linked, if I understand how it works it prioritizes one camera (the master) unless it’s not available, at which point it switches to the secondary camera (the slave). So to make it work how I want it (the user can pick which camera is used by the dash cam at any time, and the exterior camera is always on and in use by the head unit). I’m thinking I could simply connect the interior camera to the intelligent switch as the master, connect the exterior camera as the slave, and change my SPDT switch for a SPST switch that simply turns the interior camera on and off.

Is it worth also investing in a proper splitter like this one: Composite Splitter Distribution Amplifier RCA Connectors, 4 Ports
Right now I quite literally simply spliced a wire onto the data line of my exterior camera and connected it to the switch.

Thanks again! I’ve been working on this kind of stuff for a while but without any community to turn to when I ran into problem, so I’ve sort of just been fumbling around in the dark with my limited knowledge.

ya, switch camera power on/off and let the analog video switch take care of the rest.