Just got a new Garmin Nuvi 660, and the FM transmitter is terrible, I've read about trying to connect MP3 players and phones to the the phone input, and even downloaded the pdfs, but I'm still not clear on the cable connects and how it operates.

Ideally I'd like to be able to listen to the radio or cds, and when a turn is required or the phone rings, the head unit switches to the phone input and plays the output from the Garmin, then once that is done it switches back to the radio.

It seems to me that is how the phone input should work. Stereo / mono doesn't matter to me.

Is that how it works? What do I need to buy?