I've been looking for the same thing.
Here's what I've found so far.
A guy on the edmunds rx330 townhall discussion list has called Curt Manufacturing -- They make a trailer hitch that will fit the RX330. However, they aren't making the wiring harness yet. I guess its in design right now but they don't know how long its going to take.
Here's the relevant section of his post:
The Curt hitch, part #12530, is available from Drophitch.com for $111.98 and is the best price I've seen yet, courtesy of Cotmc. I spoke to someone from Curt Manufacturing and was told that they are currently trying to put a harness together for the RX330. Their number is (877)287-8634. Maybe you can get more up to date info or get on their notification list.
He went ahead and bought the Lexus hitch since it includes the hitch and integrated wiring harness. -- The wiring harness for the Lexus hitch runs through the tube to a recepticle mounted on the hitch. He said it was around $265 from his dealer. I've asked for a part number from mine and they told me PT22848035.
I've also asked for pictures of the hitch when he picks it up which should be in the next few days.
I'm going to hold out until I see what the Lexus hitch looks like. I've already found the wiring harness connector under the cargo bins in the back. Its under the right side cargo bin of my AWD RX330. I think I removed about 4-6 plastic plugs to pull the bin out.