will a switch work?

I'm pretty sure this will work, but i need more opinions.

I'm installing an overhead into a 2008 Ford Explorer with the Microcoft sync system.  Instead of going with an FM Modulator i'm going to hook into the factory aux in located in the center console.  I found my wires already for left/right and ground (thanks "I am an idiot"...saw it in another post).  Here's my dilemma...If i tap into the aux in wires, the overhead will be heard through the speakers of course, but what if they want to hook up another device at the same input?  which unit gets priority over the other, and if i hook up a switch to the negatives of the a/v out of the unit it should kill the sound from the o/h, but will i get any interference? 

thanks for the responses...and i'm sure somebody's done this before... 

thought i'd let you guys know how it turned out.

It did work the way I intended it too, with the exception that I had to hook the switch to the two positive wires coming from the o/h unit.  There is no interference or engine whine whatsoever and the switch works flawlessly if they were to plug anything into the actual mini-jack input.  It's actually really easy to do if it comes up again.....and I love customers that tip.!!     : )