93 Lexus with Nakamichi

We are having to replace the factory h/u in a 1993 Lexus GS300 with the Nakamichi system to a flip out screen. It has a single 12" driver under the rear deck. I have a harness for a Toyota that has direct RCA low level outs but this harness will not even power the new h/u up. What needs to be done to change the h/u? It has speakers in the doors with small ports. Help!!

Did you connect the amp turn on lead in the harness?  That should power up the factory amp.

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My ex once told me I have a perfect face for radio.

Thanks for replying! We found the amplifier under the changer in the trunk. It had standard Toyota wire colors on the output and input plugs on the amp, so we just bypassed the factory amp and added a small amp on the factory 12" sub under the deck. We couldn't use the factory amp because I was told by metra that the turn on wire for the amp was a data connection and not a 12 volt source turn-on. One thing that still boggles my mind is why in the heck no-one makes an install kit for this car?? I had to grind down a 2002 model ES300 kit an iso mount the aftermarket h/u. Go figure? Thanks for the help! Maybe this information will help anyone else with a Lexus GS300 with Nakamichi. It rocked when we finished the install. Surprisingly the 12 in the deck with no box was really responsive and clean. Keep it loud!
~~Derl~~