94 Lexus ES300 stereo

Ok, another fng here, so please be patient w/ me!  I recently bought a 94 Lexus ES300 for my daughter.  Beautiful car, but the stereo is surpisingly weak.  It seems like the stock reception and quality of the Pioneer head unit is ok, and I like the way it integrates into the dash.  The sound is very weak.  It seems like the speakers may be ok, looks like the fronts are 3 way separates.  I'm guessing the rear deck 6x9's will have to go.  The first thing I want to do is replace the stock amp, under the front passenger seat, with a 4 channel Kenwood amp that I removed from my old truck.  It has both RCA and Line level inputs and is rated at 25 watts x 4 channels.  I also would like to run a cheaper Phase Linear amp off of the RCA outputs on the Kenwood to power a single 10 inch JL Audio sub, from the same truck.  My question, are there any harnesses available that will allow me to plug into the factory harness for the stock Pioneer amp under the front seat?  The Kenwood amp is about 4 times larger than the stock amp, so I know I'll have to locate everything in the trunk.  If no harness is available, does anyone know where I can find information about the wiring colors for the factory harness?  Any help or suggestions are greatly appreciated!

i dont know if there is any adapters made for the factory amp but there is complete wiring diagrams on this site go to diagrams and search for the make model and year that will help are you going to run the JL 10" sub off another amp ? if not i would suggest running the rear deck speakers of to of the channels and bridge the other 2 channels to the sub and keep the fronts running the way thay are that would make the wiring much easier let me know if you need more help and what your plans are1974 ford pinto 4 15" punch Z power punch bd 1001 pioneer DEH-6700

I definitely want to keep the functionality of the balance and fader controls of the factory head unit.  I do have a separate amp for the sub, and even though it's a cheapie, it can be configured to only send the bass frequencies to the sub and bridged, is rated at something like 125 watts.  I figured the Kenwood at 25 watt per channel is ok, temporarily, for the stock speakers and even with 2 channels bridged, wouldn't be enough for the sub.  The Kenwood amp has RCA preouts, so running the second amp is not a big deal.  I want to keep this as simple as possible, and appreciate the suggestions!