I just finished installing an aftermarket Class 3 hitch on my wife's 2009 RX 350. The hitch is a U-Haul brand, and its made by Curt. I got mine on eBay for $130 including shipping. It fit perfectly; you remove the two black tow brackets attached to the underside of the car, and the six bolts on the hitch line up perfectly with the OEM nuts which are already welded to the car.
The OEM wiring harness can be purchased as a separate part from the dealer (Lexus part number 08942-48870-BB). I got mine for $44 including tax. Installation instructions can be found earlier in this thread in a post by GDixon and are accurate.
Installing the harness is a breeze. It helps to have a nylon body trim tool to pop the nylon clips holding the various trim and trays. That and a 12mm socket wrench and a 12mm open end wrench is all you need. It took me about 45 minutes to disassemble the trunk floor bits, install the harness, and reinstall. The wiring harness comes with clips to also attach it along the OEM hitch body under the car -- since there is no provision for these to attach to my aftermarket hitch, I just looped the wire over the bumper mount and zip tied the plug to the hitch. Nothing hangs down below the bumper valance, and there's no stress on the wire at any point.
My final tweak was black electrical tape over the UHaul label. Everything looks very neat and black ... after a couple of days, my wife didn't even notice it on her car.
