I have a 2005 xj8L. water keeps coming in at the rear floor vent on passenger side right at the b post and standing in the rear floor well. Any ideas of where or what? front floor area bone dry on passenger side. just soaking rear area on the one side.

 

The first place I'd check is the sunroof gutter. Check by opening the sunroof, and pouring some water into the gutter and see if it drains out in a timely manner. The drain lines from the gutter (one at each corner) can get clogged quite easily, at which point the water will overflow and come down one of the pillars. I had water coming down the passenger A pillar and filling up the foot well, as the way I normally park my car, that ended up being the lowest corner. Fortunately for me, the few times this has occurred, it has been the front drain lines that were clogged which are easy to get at. I'm guessing the rear drain lines are not very accessible.

 

I saw a neat trick on TV the other day for finding water leaks. They sat a smoke machine - like they use at parties - inside the closed car and watched for escaping smoke. The smoke is non-toxic and leaves no odor or residue, so there is no damage. Probably could rent one at the local party store if the sunroof isn't the source.

 

The smoke is going to float past gutters, or other measures that will normally redirect water.