It's the pin switch that tells the trunk light when the trunk is open, much like a dome light works with the door jam pin switch, it sits adjacent to the trunk latch. This one is badly mangled and has at least two damaged parts. No salvage yards in this area have any LS400 vehicles except a stripped down shell at a U-pull it. I have no Lexus parts or support here, this is GM country
I found that Park Place Lexus in Tx has excellent prices, other than that just ebay. I might be to OCD about getting it just as OEM for a lasting fix. As far as information and searches its this forum and Google, which often leads back to this forum. I have so far been able to remedy a lot of little issues and upgrades/mods with searches alone.
In the absence of a pic to see what it should look like I'll recreate the mount and fab up a cog and put it in a position that works and hope it lasts. Near as I can tell the fail had to come from someone trying to force the trunk to latch closed with a blanket or something caught in there. Based on the damage they were very determined.