Hi Guys,,,
My IS200 is constantly plagued with Damp/Condensation in the rear Window.
One thing to possibly check is under the Grey Carpet Covered Plastic Honeycomb/Mesh Cover on the inside of the Boot Lid.
Upon taking mine off there were Water Droplets all across the inside of Boot lid as well as in some of the HoneyComb mesh holes of the cover.
My Spoiler Bolts/Nuts on the inside of the boot lid are all rusted from the water. We dried the inside of the bootlid with a Drier and this has definately
helped with my rear window.
You have to take off the black plastic Covers on the Boot Lid Arms (Sort of push clips) first before prying off the clips that hold the cover to the bootlid.
Im suspecting on mine Water maybe getting in through the holes that are in the boot lid from the Spoiler as the rubber Seal on the boot itself seems fine.