Today was a good day for progress- I finally got to the bottom of the last audio problem. I isolated all of the inputs from the audio amp except for the music, which works fine. The circuit is designed to use resistors to control the volume level of each channel. I gathered from the directions that anything from 30-500 Ohms would be required. When I connected the Dynon audio to the iso amp, it was just super loud, to the point of being very distorted. I figured I would give the circuit a chance by using higher resistance, without regard for any norms about what should be required. I stuck in incrementally higher values until I made it all the way up to a surprising 3000 Ohms. At that level, the circuit worked fine. I reconnected the Garmin nav output and repeated the same strategy. You can imagine how surprised I was to find that it required 68,000 Ohms! With those very high value resistors in place, everything works exactly as it is supposed to, which is a big relief.