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I finally got the stock sub working with my aftermarket stereo. No audio integration harness is needed. All you need is a RCA to speaker wire cable and tap into the Sub + and Sub - wires (black wire and white wire). I was following the diagram http://www.intellexual.net/faq/lexusstereo.swf for Socket D and it lists the Sub + signal as D1 and Sub - signal as D5 ; however that did not work so I thought maybe I was looking at the diagram backwards. So I tried tapping the speaker wire to D4 and D10 and heard the subwoofer. So it appears you need to tap into D4 and D10 instead of D1 and D5. Great day. Got the subwoofer working and installed the double DIN Metra kit.
I really appreciate you posted that information here because I never did figure that out and gave up. I have reason to go back and work on it again. You may want to bring that infor to the admin on intellexual.net, they will post corrections.