I have a Sienna that had no wiring or hitch.
Got the Curt Class III, fits perfectly. Made the mistake of trying to do it myself, but couldn't get the bolts in--too much rust. Took it to a local hitch place, they did the job, gently berated me for stripping out at least one of the nuts (they had to recut the threads), but all in all it's a perfect installation, and I'm very happy with the Curt.
As to the wiring, the best place I've found for information and instructions is etrailer.com. They have wiring harnesses for most vehicles to tie into the back lights and make a 4-pin connector. I did all the wiring myself (never did ANYTHING like it before, so it goes to show you no special skills needed
) The 4-pin was simple. Running the wires for the brake controller and power for the Bargeman was a larger project, but it got done.
I chose to build my own "install kit" because I wanted a particular housing for the 7-pin socket, but if you want, etrailer.com sells a pre-made kit (wires, connectors, breakers, everything you need) to complete the task. There are videos on the site, too, to help with the installation of both the 4-pin and the 7-pin wiring. Plus, the people on the phone are extremely helpful.
You'll also need a brake controller. I chose the Tekonsha P3, but many go with the Prodigy.
I'd suggest you start there.
Examples:
Hitch:
http://www.etrailer.com/pc-H~13594.h...cleid=20089847
4-pin wiring harness:
http://www.etrailer.com/pc-VWH~11844...cleid=20089847
4 to 7-pin wiring kit:
http://www.etrailer.com/pc-BC~ETBC7.htm
I also got a cheapo trim removal kit from Harbor Freight (like $3 for four nylon mini-prybars) which proved to be invaluable for hiding all the wires without any damage to the trim.
Marc