I have a 2000 ES300 and couldn't find a decent aux in kit anywhere for my factory head unit. (premium sound system) I did finally find a guy in California through his ebay store to do a custom install for me. I had my head unit pulled and sent it to him. Looks professionally done and the quality of sound is amazing!
His name is John and it only costs $55 + shipping. Here is his website
http://www.factoryradioservice.com/
For anyone who has an earlier model lexus and want to keep the factory unit look, contact him and he will set you straight. factoryradioservice@gmail.com