I've built a simple adapter to mix multiple line-level AUX inputs to a single input on my home stereo, and it's worked fine. Incredibly simple, just some female RCA plugs and resistors.

The only reasons that you might not like it in the car is:

- You might run into ground loop noise issues. But honestly, a complex switching system is going to be MORE likely to induce this than a simple resistance mixer would.

- If both devices do not have a volume control for their aux outputs, then you would not be able to control their relative volume. Maybe not an issue if you plan to mute the tuner by hand when you get a call.

I seem to recall that it was the EmpegBBS that told me how to make a resistance mixer, let me see if I can find the thread.