I also have a 1992 ES300. This is what I used:
http://www.carstereohelp.com/stereoremovalLexusES300StrR.htm For the most part, these directions are right. First remove the clips inside the vent plenum. Then, if you look on the sides of the plenum, you'll see two holes (one on each side). (DON'T do the "...major pressure to the side..." like it says!) Using a bent coathanger with a small hook on the end, pull gently while prying along the TOP seam of the same plenum, same general side. The plenum should release on that side. Then do the other side. It's easy.