I have had the SOS error ever since my Kenwood KVT-516 install. The SOS error comes up sometimes, sometimes not, but the triangle with ! always shows up -- which indicates an error.
My SOS still works as I tried it and got someone on the other line and had a conversation.
I then had the OEM head unit relocated to the trunk, where a spare tire would go (have run flats). CAN bus, Fiber Optic cables etc... are connected as described.
I can see light so it appears no kinks and it is making a loop.
The power for the headunit is a constant as someone described, so that was done.
Long story short, this didn't work for me. Not sure what went wrong, I still have the SOS error.
The headunit is connected so can do updates if needed.
Also, there is a fiber optic cable in the trunk area too with some other hardware. Can we grab the fiber optic from there and loop that to the head unit?
Any way, I've given up and can live with the little warning light for a HD radio, bluetooth, Navigation, rear view camera, iPhone etc.. Kenwood unit.