The Electrical Assy Manual specifies a C3SB-14A163-A Retainer for all models.
It attaches to the wheelhouse fin over the black plastic luggage protector.
It's also the way the fuel tank sensor wire is held in place under the car.
The Manual specifies the use of a plastic shield retainer (C7ZB-14A088-A) under the car, but I've only seen the use of the C3 retainer in the wild.
Here is a straight on and 90* view.
Note in the straight on view that the top 60% or so of this side is rubber coated. The center finger is coated on both sides, to protect the wire. This example has been P/Oed, so the rubber is hard to make out.
Note that this example is out of the GT350 in that location.