Thanks for the help. I have gone through the PDF "Interfacing the Zumo 550" on autocomm's website and I do see these 2 options. Maybe the recommended, pricey option is the way to go; I didn't realize how expensive getting a Zumo was going to be. But, let me try and understand the alternate. Besides, that bluetooth module under the seat might give me prostate cancer.
The reason overrides need to be disabled in the Proaudio, as I read (note, not understand) it, is that to put the input into a stereo input (to get stereo music), the audio from the Zumo must go into a low-priority, stereo input. The instruction says Aux 2. It seems that the stereo inputs are low-priority, and overridden by other inputs.
But what are my other inputs? Audio from the rider and passenger mics. There wouldn't be any others (unless I run music out of my phone into another Proaudio input, but let's not get too complicated yet). And if either passenger talks, I would want that to override the GPS, music and phone, wouldn't I? If I just wanted to drown out the pillon, I'd disconnect the cable (and wouldn't tell her). I'm sure the answer is no, or everyone wouldn't be saying you need to do this.
When I talk on the mic, will the Proaudio output that on Aux 2 as well, so that jack is both an input and an output, just like it is on my phone? It must be, because part 1299 has one plug for Aux 2, and splits it into to separate plugs for the Zumo.
Note that part 1299 does not seem to be available from the website. Maybe that method has been "discontinued". I haven't heard anything back from Autocom yet.