Car Stereos: can i add another head unit, rca cables, stock speakers

Question
I have an 08 ford expedition with factory sirius sat, and SYNC. I want to keep all of that. But I would like to install another head unit in the glove box to power some subs in the back of the truck. can I just tap into the factory speaker wiring for the other head unit. and just turn that radio on when i want to use it with a toggle switch.

Answer
Hi,

If I'm understanding you correctly:  the only reason you want to add a second head unit is so that you add subwoofers to the system.  Is that right?  If so, a dual head unit setup seems like the wrong way to go about it.

To answer the question, you really can't just add another head unit and wire it directly to the stock speakers.  The trouble is, when you have two head units connected to the same set of speakers, you effectively have the two head units connected directly to each other's outputs.  You'll end up damaging one or both of the head units' amplifiers.  The only safe way to use two different decks with the same set of speakers is to use relays so that the speakers are only connected to one head unit's outputs at a time.  This can be done, but it involves a fair amount of wiring because you'll need to switch at least eight different wires to a different source at the same time.  You'll also have to find a splitter for your antenna, or install a second antenna, if you want your second head unit to pick up radio stations.

A better option is just to connect your subwoofers' amplifier to the factory radio's outputs.  This can be done with a device called a line output converter (LOC).  The LOC is connected to your factory speaker wiring, and it converts the audio signal to a low-level output that can be connected to any after-market amplifier using RCA cables.  You can even install a toggle switch on your amplifier's remote wire, so that you can turn your subwoofer system on and off.

Hope this helps!

Brian