I have the tow hitch and added 2, PIAA 50w backup lights (they come as a kit with the relay and wiring). I chose not to include a on/off switch in the circuit. With the dark window tint and my old eyes, it's much better, but not wonderful.
Remember not to add any additional load to the current backup light circuit, just use that power to trip a relay. I tapped into the rear 12 volt outlet circuit for power that will go to the lights via the relay; going directly to the front fuse box for the 12v power to the relay is technically better, but I didn't want to rip up even more of my new luxury car.
The dissembling to gain access to the wiring took a long time, the wire insulation is thin and tough and several wires in the harness have the same color. I ended up using a probe on my voltmeter which has a very fine pin to validate I had the right wire before I added the wire taps. I hate wire taps, but could find no easy way around them, the connectors for the circuits are near the amp for the sound system, and I was concerned about damaging the interior just to get to them. Instead, I accessed the harness in the rearmost hatch cavity, on the floor, below the 12 volt outlet, near the trailer htich connection. I fed the wires through the hitch wiring grommet.
The wiring harness and circuit information as well as some interior panel disassembly can be easily downloaded from the toyota/lexus online-manual site. In a hour or 2 you can download tons of PDF files to save on your pc. It only costs about $10 for one day of access.
I have the lights attached to the round tubing of the hitch using stainless steel hose clamps. I put black rubber under the light bracket and hose clamps to prevent damage to the hitch paint. Not elegant, but functional. I aimed them a bit above horizontal so I could see poles, signs, etc that the rear camera does not make obvious.
I sent Lexus a note that the weak BU lights were a safety hazard, but never heard anything from them. I also posted the issue on the DOT vehicle safety site, you might want to do the same.