Also both SM-10's and RD are powered by separate circuits from the EZCan II.
Why separate circuits? Those things draw so little, you could put 10-20 of them on one circuit. Plus, you need the SM10 working to hear the RD. If you lose either one, you're out of luck as far as the RD goes. In addition, I don't have a spare on my ezCAN. I'm using channels as follows: aux lights, horn, tail/brake, and accessories where the accessories as the RD, the SM10, the Clearwater Betsy's with Shrodie's, and a 12V outlet in the trunk which is used for charging a tablet or other small electronics..I’ve done similar: each SM10 is powered from separate EZCan II outputs. My Fluke meter tells me that they share a common ground, so I chose the route of leaving the audio ground to one SM10 floating as a precaution, but your experience suggests that may not be strictly necessary.