i builted a amp rack in for the back.....i took out that dumb ski bag thing from the rear and mounted my rack there....from my past insall days i bypassed the factory amp....one side of the amp is signal in(from the radio)..the other is signal out(to ur speakers)..i used my old trusty speaker tester..a 9volt battery with a wire comming from each lead(i solider them in).....and popped them to find out which wires went to which speakers.....now....lexus make all speakers in that dayum car common grounded....meaning that all the speakersgrounds are hooked together at that amp plug.....if u look on the pins of that plug....5 is for front left...6 is front right...13 is rear left....14 is rear right.....(on mines atleast)..remember..they are common grounded...and the front dash speakers..those dayum things work off the radio by themselfs i thing....but u can drop the wires right there ..run new ones..if u remove the speakers and look in the hole...its a str8 drop for the new wires to ur amp...........peace