pgamboa

So I purchased some power folding mirrors from Boost Auto Parts. I have a 2015 Chevy Silverado 2500 HD LTZ. This is the early model. This truck came with tow mirrors that were not power fold. After installing the mirrors and power fold switch (Haven’t modified the door harness, or ran the rzy harness yet) the mirrors have the turn signal working on both as do the normal operations of the mirror ie tilt and movement. What doesn’t work just on the passenger side the power fold doesn’t work. The driver side power fold works but not the passenger. Boost Auto tells me to splice into #4 and #18 and run a wire from the mirror on the driver side to the #4 and #18 mirror connector on the passenger side. Not sure why this needs to be done this way. Any info on why?