Motorcycle Intercom help needed please...

Hello good people,
I ride a Honda Goldwing motorcycle with a radio/tape combo and also built in intercom between front rider and passenger.
I have a 5 pin din socket on the bike into which I plug a 5 pin din plug which is connected to two earphones and a mike in my helmet. This enables me to listen to music and also talk to my pillion passenger.
Now my question.
How do I connect my satnav or mobile phone to this circuit without damaging current setup on the bike ie prevent feedback back into the radio/tape. Will diodes suffice?
The Satnav "speaks" directions and I would like to hear directions through existing earphones. How can I connect the Satnav to the circuit?
Also I would like to connect a walkie talkie to allow inter bike comms and use existing mike and earphones.
I am a complete novice so please go easy on me!
Any advice and help will be appreciated.
Thanks in advance.
Barry.

Diodes cannot do the job since it is AC.

I'm afraid I don't have any good news here. As you figured you can't put the new source in parallel with the intercom output. Technically it can go in series but there are actually grounding problems that make this sort of impractical. Maybe if you had an audio isolation transformer that handles headphone levels you could put it in series.

Well a circuit could be made to sense sound on the second source and a relay automatically switches to the other source. This is fairly complicated.

You really need an intercom with an external source.

Thank you for the reply Oznog.
I thought that there may be more to it than meets the eye

Barry.