Thanks to those who have taken the time to reply. The rattle happens with the seat down and the trim removed. I went ahead and pulled every loose item out of the truck, and removed the tonneau cover just in case. Really, it sounds to me like it's coming from inside the rear wall. If it's at all...
Hey crew, based on all the great info on these forums I've been able to upgrade my stock speakers in my XL hybrid, and this past week got a Kicker HS10 all wired up. It sounds fantastic! ... when I have it sitting on the rear seat. I followed the video from Andy's Amateur Adventures and got it...