These are different problems
1) You want to get rid of the errors . So the descriptions of the way to set
up your OEM head unit that are in the relocate the head unit thread. There is something you need to understand about the MOST BUS ring technicaly that picture of the fiber optic link between the amp and head unit in the L7 diagram is not correct. The Most bus is actually a ring bus with and in and out line at each module(fiberoptic in out). So it will go from the head unit to the amp to the Bluetooth to the SOS and come back to the Head unit. In order for it to work there can be no break in the loop. If there is everything that is on that loop will not function and or produce errors. Now I think people by pass and remove the L7 amp as part of the procedure in the relocation procedure.
There is a second way to get rid of some the errors by programming the modules you don't want out of the cars list of options, but I don't see people use that
a lot on this board.
2) Getting sound to your speakers.
So now you need to get a harness that will route the pre amp signals to the Trunk area in a analog format for and amp or processor. (As I have been told
that cars with the optical connection do not have wires for this) Then you can feed the outputs of your amp/processor through the factory harness for the speakers at the L7 amp location. You should get in touch with Technic for such a harness and the specific information you need .
The kind of systems most build for a L7 system because of the 9 channels it has usually start with and ms-8.