Looks like the bat cable runs down the pass side...
If I'm running RCA outs, do I go down pass side, middle, or driver's side ?
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How to route rca audio in M5 ?
The easiest way to run small signal cables is to carefully pry up the door sill cover plates. Once you have these off for the front/rear doors you can lift the carpet and route the cables along the intersection of the floor and the sidewall. If you route them down the left side of the car you should be able to get into the trunk via the same grommet the lighting harness takes into the trunk.
Rich
Rich