Electronics 2000

Hi
Does anyone know if there is a commercially available device that can run off 12 v or less that can take the speaker connections off a sat nav, output connection off an mp3 player and player the mp3 player but mute it when the sat nav gives a route direction, obviously the device would have to be plugged into the aux connection of a cars stereo.or put me in the right direction on how to make one.

I dot know about a commercial device... You need some sort of signal from the satnav... If you can tap that signal you can use it to switch switches... Then you need a signal to switch it back again.
What sort of satnav do you have?
What are you like at electronics?

I don't know of a commercial device... Acccccchhhhem.

Obviously the signal from the sat nav would only be the actual start of the navigation details of the route which would continually stop and start the only way would be to take the wires off the internal speaker of the sat nav.Then when the sat nav instructions had ceased the mp3 player could continue playing, much like the current sat nav I have (which I want to replace but the new one does not have an internal mp3 player) the output from the internal mp3player mutes when a direction is given out. I can follow circuit diagrams etc and construct electronic projects , I understand a little bit on how electronic components work etc.

It depends on the speaker set up... If there is an amplifier circuit which switches on when directions are announced then off, you can use that as the signal. How ever if the amp is always on it is not that simple, pause between words would also be a signal which would switch when you don't want it to.
The wires must come from an amp not the speaker.
Can you use a multi meter to find a signal?

The switching part is easy if you can find the appropriate signal.

There might be a speaker earth and it might switch on one off when needed...but I doubt it...worth a look though.