If you also power the sat nav and any other devices such as a phone/mp3 player from the bike, and feed the audio from those devices through the Autocom, you make get some interference / whistling / squealing. It took me a while to work out the problem, but when I re-read the autocom instructions I discovered I needed another small unit to plug the audio devices into, before they joined the Autocom. The sound is completely clean now. There is more info on the Autocom web site. It's about £25 for each channel.
My unit is also in the tail area, underneath the saddle. I have fitted an auxiliary power box under the saddle from which you can take around 6 feeds for different devices. You can choose whether to set these to turn off when the ignition goes off, or stay on. I also have an isolation switch in the system which will turn everything off, even if the ignition is on.