First of all hello!

Secondly... My Question!

I need some advice. At the moment I am hooking my Sat Nav or Ipod up to my car speakers using a FM Transmitter, like an iTrip. This means that when using my Sat Nav (Garmin Nuvi) I have to use the to play my MP3's.

So here is my question, when i get my new car I want to install a dock for my IPod, but I want another 3.5mm aux in jack to plug into my Sat Nav which, when my Sat Nav gives an instruction or warning will override the IPod until the instruction is over then return to the IPod.

Is there a device which will play one signal but on recieving a signal from another source will switch to this source until the signal ends, or, Can i make one? I can make a circuit board!!

Cheers
Ed