There is a large connector block above the fuse box where common signal wires are joined (reverse signals, ibus and ground). The brown wires are ground.

To get to it, remove the entire glove compartment (8 screws), release the fuse box from one of the hinges (unhook) and drop the fuse box as low as possible. The connecor block can be unclipped or you can remove the ground bank of connectors.