I am putting together a car audio system and replacing the factory stereo with a new aftermarket head unit that has a single aux input. The vehicle comes with a single OEM aux input jack in the dash. We’re looking to add a second aux input jack in the rear passanger area so passangers can play and control the vehicles music from their device (iPhone/iPad/Android/MP3 player/etc.).

In regard to the exist OEM aux input jack in the dash, the factory harness that sends the signal through the cars wiring to the OEM stereo will be bypassed and only the physical plugs will be used so that we can send the signal through our own route.

Essentially, I need to be able to combine both inputs from the OEM aux jack and the NEW aux jack (being installed in the rear passamger area) in to ONE 3.5mm plug that will go to the single input on the new aftermarket head unit.

In my initial research, I’ve found that there are “mixers” I can buy but all the ones I’ve found have manual switches that need to be switched to change sources. We need both inputs to be active so that we don’t need to flip a switch between the sources.

Also, it needs to either be a “passive” solution OR an “active” solution that can be powered by 12v DC, given that this will be installed in a vehicle.

Is there any product that I can purchase to accomplish what I have described here?


I am also willing to build my own custom wiring harness to accomplish what I need. I can purchase wiring, plugs, etc., and solder everything together to create a custom solution.

EDIT: So I decided to just purchase a mixer (Behringer MicroMIX MX400). The power spec for the mixer is: 12v DC/150mA. It comes with a power supply that plugs into a wall outlet. I need to power it in a vehicle. Since it’s already 12V DC, I was thinking just connecting it to an unused/open circuit the interior fuse box, that is 5A switched (only active when key is in the ignition). The power will be connected to the fuse box circuit and I’ll just ground it to an exiting ground point.

Will that work?

  • \$\begingroup\$ The product is called a switch, a DPDT switch. However, in my car, I've bought a 3.5 to 3.5 extension lead. I plug one end in the dash, throw the other end into the back, and tell the kids to practice their DJ skills. No wiring modification to the vehicle, can be moved from vehicle to vehicle. \$\endgroup\$
    – Neil_UK
    Dec 2, 2018 at 5:49
  • \$\begingroup\$ this may be better suited for the mechanics stack exchange. Simplest answer is wire them in parallel and be done with it. Slightly better answer is adding a series resistor on each jack. The two signals will mix at the head unit. Next is adding a dpdt switch for manual control of which jack is connected. Further is an active system with op amps, a fixed mixer, an active mixer etc. You can under or over design this. \$\endgroup\$
    – Passerby
    Dec 2, 2018 at 5:51
  • \$\begingroup\$ See electronics.stackexchange.com/a/136134/17178 \$\endgroup\$
    – Passerby
    Dec 2, 2018 at 5:53
  • \$\begingroup\$ Thanks for the suggestion but a solution with a switch is not we’re looking for; both sources need to be active at once. \$\endgroup\$
    – Inked_xx
    Dec 2, 2018 at 21:21
  • \$\begingroup\$ Your 'edit' is an entirely new question; ask it as such instead of editing this. \$\endgroup\$
    – Hearth
    Dec 2, 2018 at 21:39

1 Answer 1


Use a switched socket in the front and arrange the switches to disconnect signal from the back socket.


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