Mixing Signals to External Speaker

[Cadence10m]

I want to combine the output from two separate devices and input both to one speaker. I have stereo speakers in the cockpit now, and want to hook one of them up to the external speaker jack on my VHF radio (it's about 2" away).

My idea was to use 4 diodes, one on each of the 4 wires coming to my speaker (2 from the VHF and 2 from the CD player). I thought they would work as "check valves" to ensure that no power from the CD player's output would make its way into the speaker jack on the VHF and fry it. A friend of mine who knows a lot more about electronics and me told me that won't work. But he didn't think a very simple circuit with one transistor would work. He said what I needed to find/make was essentially a very small "amplifier" (but with no gain). It would work is a solid-state mixer to combine the signals.

Then if I'm listening to music a radio transmission comes in, I can hear it mixed in over top of the music. If it's important and I want to reply, I'll turn off the CD player. No extra holes required in the fiberglass, and no 3rd speaker to install.

Has anyone done this? Can anyone point me in the direction of a circuit diagram? I don't mind building it, if I have plans.

Thanks for any help,

Craig