I found the 70-8112 integration harness pin connections here:

http://www.metraonline.com/downloads...NST70-8112.pdf

Pins 8,9, and 10 don't seem to match what's shown here:

http://www.intellexual.net/faq/lexusstereo.swf

Can someone confirm which is correct for the SC300 Lexus Premium Sound System?

P.S. I later found this:

http://carstereohelp.net/wireharness_Lexus2.htm

It seems to confirm that 8 is Power ntenna, 9 is Ill-, and 10 is Tel Mute

 

All this takes is a little repinning of the wires. I had to do this with my 2nd Gen GS. It is a little mixed up but compare the wiring on the Metra Harness and http://www.the12volt.com/installbay/...ard/Lexus.html.

Factory amp will be circumvented and you will have to extend the wires to the factory amp to bypass. The power connections are a little trickier, for those I suggest looking from behind the factory stereo.