I purchased a used 2015 Yukon XL and noticed that the blind spot alert icons in the mirrors were only working intermittently. I found that the bracket for the passenger side radar module inside the right rear bumper corner had broken loose and the module was pointed upward instead of toward the rear bumper corner. The brackets seem to be heat welded in several places into the rear bumper plastic and I guess this one had vibrated loose over time. With some zip ties I was able to secure and align the bracket back into place, more or less, and now both sides are working correctly. The real fix would be to replace the entire rear bumper plastic assembly which I would guess to be well over $1000.
