This site has been an amazing resource during my conversion. I figure I'd try and give back in some little way by sharing my experience in extending and relocating the LED cargo lights to my garage area.
My 2019 High Roof Extended Transit came with 3 rows of LEDs. I wanted to take the 2 rows that were in the ceiling crossmembers and relocate them to under my bed to provide lighting for the garage that would be triggered by door opening/closing (there's also the really convenient switch at the back left that kills the lights).
I extended the wiring with 14 gauge (factory is probably ~18ga). The first row - closest to the driver - was extended by 57" and the second row by 26". Cut each right behind the connector and soldered the extension in place.
I used the plastic clips on the harness to hold the wiring in place along the bed raceway. The black plastic clips can be pushed and held in wood with a 1/4" hole. The white plastic clips can be held with a 5/16".
I got super lucky in that the back row of lights hold themselves in place because the bed supports were spaced out enough to engage the clips on the sides.
Thanks for following along. Hopefully this can be helpful to someone else in the future.