How to connect 2 audio sources to one speaker?

Hello, I'd really appreicate if someone could help me out here. I have a Samsung Bluetooth Car Kit which outputs the sound though a seperate speaker. What i'm looking to do is connect it in some way to the cars owm drivers door speaker. The Head unit mutes automatically when a call comes so there would be no sound going to the speaker from it when a call was in progress. The simple way to do it in my eyes would be to just connect both sets of speaker wires to the one door speaker, but i'm concerned that the 50w RMS that the head unit can blast out would be feed back into the bluetooth control unit would in some way damage it, and vice versa when I was talking on the phone. What could I use to prevent the sound getting feed back, some type of diode or something? Like I said any help would be really appreicated as the speaker is a big ugly thing which I'd rather not use if possible. Thanks Davy

It sounds like you basically need an audio mixer.

You could search the net for tutorials on how to build a simple one yourself, or spend about $35 and buy one of these:

http://www.amazon.com/American-Recorder-AR-PMIX-Personal-PMIX-100/dp/B000M6QAUG/ref=sr_1_41?ie=UTF8&s=electronics&qid=1253115724&sr=1-41

When coming from two audio sources to one set of speakers you definitely need something to isolate the sources.

Another option is an audio "A/B" switch which allows only one input source to access the speakers at a time. That would require manually switching between your car audio and the cell phone when making or receiving a call. If that is the route you want to go, A/B switches are very cheap.