I think it is the secondary horn and or alarm siren as it is sitting in the trunk and wire codes look the same upon quick inspection. I think somebody probably cut it during one of its I would assume many towing sessions..

I wonder about that on Mercedes I have seen them both with and without all the connectors required. Sprinter van - no additional as each harness was custom built. No idea on these cars yet. Although I assume it was as you describe with things just left unplugged. I hope that is the case if I find heated or powered seats to swap in.