Unclip the holder and pull it up to determine wire colors and size. Perhaps then it can be identified. I suspect it is for the throttle and the Bentley book indicates the wrong position for it on the diagram.
According to Bentley the wires to fuse 55(electronic throttle control module) are red & red/gry/yel. Gotta be it, it's the only 50a fuse in the schematic.