drwankel wrote There's a simpler method to allow you to root cause this, assuming the car was working correctly before hand.
Unplug the fiber optic connection on the mr12volt unit while the car is on. Do you see red light coming out of the cable? You should. If you don't, its not the unit. Assuming you don't, work your way backwards from the spliter. Is there light coming from fiber optic connection on the vehicle harness? There should be. Remember, MOST is a loop. Each device sends the signal to the next device in the loop. When the loop is broken, you just need to walk back to where you lose signal.
I've been having an issue with my install where all the CD slots are marked as unchecked and my CIC headunit is not recognizing my mr12volt either. When searching through this thread, I think this may be my problem. 2011 CIC w/factory combox and HiFi speaker system FWIW.
I've disconnected/reconnected everything going in/out of the FO splitter and believe it should be correct. I'm seeing a red light from the FO connection when unplugging the splitter from mr12volt and mr12volt itself is also showing a red light. I can also hold down the CIC dial for ~10 seconds and watch mr12volt reset itself:
However, I'm not seeing a red light from the OEM green/black FO which I believe is what you meant by "there should be a red light coming from the fiber optic connection in the vehicle harness". I'm also thinking I broke the MOST loop somewhere, but my original CIC unit is working as normal right now (full sound/audio) even with the mr12volt connected, so I'm even more confused, as there wouldn't be working sound if I had broken the MOST loop (correct me if I'm wrong, still learning!) Maybe it's the heat of being in my garage for the last few hours - I'm understanding that if vehicle harness isn't showing that red light, then it would mean the MOST loop is broken and I shouldn't be hearing any sound.
I took a risk in not coding prior to install after reading that mr12volt switched over CDMM_CHANGER_3 to "mp3_cdc", but after reconnecting the OEM harness/wires and checking, it is indeed coded over correctly too. Though not sure if it was actually mr12volt that changed it or if it was already done prior.
When searching for my issue in the thread, seems like some people have had similar issues with i.e. iPod connectors left in from old owners which I'm worried may be the case, but I'm struggling to find any random other cables right now under/behind the glovebox.