I'm trying to find a way to remove the rear deck panel to get to the speakers. I tried going in through the trunk but it's sealed. I'm getting a resonating rattling noise from underneath the black felt cover that holds the speakers. Dealer can't replicate it because it only does it under multiple bumps. I've searched but I can't find a how to. any help would be appreciated.

There is a diagram here that might help: http://www.toyodiy.com/parts/p_U_2006_LEXU...ETLHA_6403.html

It's often necessary to remove the rear seat back from a car before removing a package tray.

Are you sure the rattle is coming from under the package tray? If you haven't already, you could use a cheap automotive stethoscope to make sure. You could even put your wife/girlfiend in the trunk to listen for the sound!

By using a stethoscope I've sometimes found that rattles and buzzes were coming from a different place than I first thought.

"You could even put your wife/girlfiend in the trunk to listen for the sound!" roflmao
Come on honey, just climb in the trunk. 1990LS400, your wife\girlfriend must be a real trooper if she would do something like that for you. Thanks for the chuckle.

Well the rear deck has the subwoofer in the center. When I pound on the plastic subwoofer grill with my fist I hear the resonating sound I hear going over a bump underneath. Something is obviously moving under there, I just don't know what. I thought that removing the seat back would help greatly but I don't even know how to do that.

Lift up on the lower leading edge of the rear seat cushion (down by the carpeting) and see if it is held in place by a spring friction fitting and pops up and out like the rear seat cushions of most Toyota/Lexus vehicles. Once you remove the seat cushion, you will see at least some of the bolts holding the rear seat back in place.

Look around on the previous web site I provided. There are photos that show a little of how the rear seat is held in: http://www.toyodiy.com/parts/p_U_2006_LEXU...ETLHA_7102.html

If the seat back doesn't come out after removing the obvious bolts check for hidden ones under velcro flaps, etc. Sorry, I'm not familiar with the IS but there are lots of similarities in the way these cars are screwed together.

I've removed the rear seats of Toyota/Lexus cars many times -- it's usually easy to do.