This is an optional harness that allows you to re-configure the flashing of the centre lights control
The GX-W and the GX-WR need the X-Harness to control these lights.
Why do you ask? Because these lights have two functions and so the X-Harness is a dual function controlled system. These lights have a center reverse or cargo light, which is controlled by one of the switches and the second switch controllers the flashing lights in the other section.
Goes with sku