Alonan, These Sc vendors may have the parts you need. Click under used parts for first link. http://www.randjdistributing.com/. http://www.clublexus.com/forums/sc400-300-classifieds/591580-misc-parts-on-the-shelf-make-offers.html
As far as your radio system...it could be a number of things; head unit, amp, speakers could also be blown, stock speakers are cheaply made.
http://www.clublexus.com/forums/lexus-audio-video-security-and-electronics/109384-speaker-install-how-to-with-tons-of-pics-sc3-400-a.html#post802171