Does the BCM have a ground wire? I can't find any only the red thick wire for power.
Yes ... The BCM is grounded at the same point as the DDM, G302 on the lower left B pillar. A single wire runs from that ground point to the rear fuse block. It splits there and runs up through the rear fuseblock through that green flat ribbon cable at tracks A11 and B11. Inside the BCM circuit board these two tracks are rejoined/soldered together. They ran two tracks only to distribute the load carried by the two tracks.