Yeah, been there too. The rear body mounts are part of a boxed in section between the rear kick panel of the rear seat, and the outer body shell. This creates a cavity where water collects from coming in the rear side vents. There is supposed to be a drain hole in the bottom of the cavity, mine was plugged with accumulated debri, hense the rot. I ended up cutting out all the rusted parts and welding in new metal. I cleaned the area inside as much as possible first, and sprayed rubberized undercoating into the cavity once it was dried out. ( before welding it shut.) It's kinda weird, but only my pass rear and drivers front body mounts were effected. It's a pita job, but worth doing if you plan on keeping your rig, especially if the rest is in good shape. Other areas prone to rot are under the front mats, especially the pass side, and under the battery tray. Check those areas too. Welcome aboard the forum.
