Found a Lexus/(pioneer) CD changer adapter made by www.autotoys.com. Go the their website and do a search for "Lexus Aux input". The unit was engineered by them and replaces the CD changer in your Lexus. Choosing "CD" in your Lexus stereo, actually selects the AUX adapter/signal.
The cost is $99.
LEXUS AUX INPUT CONVERTER (1998+ ES,GS,LS,RX)