I am having a tough time with my Nakamichi stereo. Only sound I am getting is from door tweeters. head unit and power ant. both work, just no sound. Checked the audio mute (orange clip under rear seat) for corrosion and water, but nothing. Any ideas on what the problem might be?

 

I would say blown speakers, do a test on the rear speaker to confirm. Take the trunk lining off and disconnect +/- and hook up to a cheap 2 inch speaker. DIY steps for the rear below. The Front speaker DIY is in the Sc 300/400 audio section.
http://us.lexusownersclub.com/forums...howtopic=65329

 

There could be water under your amp (in the trunk) which could be causing the system to be acting funny. I had water under there and it caused there to be no sound at all.
The amp is underneath your cd changer, just start taking screws off and you'll get to it. Dry the amp out, and clean out the water underneath the amp (there will be a good couple inches of water) and that might fix your issue.