To route a powercord from the battery to a CB radio on the driver's side of my 2012 MKX, I used a pathway that exists beneath the upper rear of the front fender.
The trim piece that is attached at the rear of the front fender, just forward of the door's side mirror, is retained by three plastic push-pins. Carefully prying the trim piece off reveals a small opening in the rear edge of the fender.
Next, at the outside end of the cowl top, there is a triangular black rubber cap that can be pryed upward at the front edge and rotated, revealing the underhood-side of this pathway.
I fished a piece of coax cable from the front, through the pathway, and then taped my powercord to the coax. Pulled the coax forward carefully, since the pathway is a bit tight. Once the power cord was through the pathway, it could be more easily pushed-pulled through to adjust the length and to connect it directly to the battery.
The fender trim piece was pushed back into place on top of the powercord once the underhood wiring was complete.
I routed the powercord into the cabin by pulling back the front vertical weatherstrip-trim strip outboard of the instrument panel, ran the cord behind it, and pushed the strip gently back into place. Wiring tucks nicely beneath the edges of that strip for additional routing inside the cabin.
The powercord is invisible when the door is closed and the installation caused no additional wind noise.