I found this by searching pictures of "Lexus LS400 trunk hinge wiring" within 30 seconds.
Follow the hinge back and you will see the wiring loom concerned wrapped up in black tape carefully remove the tape to expose the wires and then check each wire carefully, IIRC the white wire is the normal culprit then follow the repair instructions provided on www.lexls.com
You will probably find the easiest way to work is to lie in the trunk and have a decent flashlight to hand.
http://s731.photobucket.com/user/rojo412/media/Stereo%20Howto/DSCN0018.jpg.html