has anyone wired a trailer for their prius

And if you use a luggage tray, it's a good idea to use trailer lights on it.

 

For anyone else with a Prius OLDER than 2004, the instructions at
Toyota Prius: How to Install Trailer Hitch Wiring - PriusDIY.com
also work for the 2001-2003 models, with two slight changes:
1) No need to remove the (non-existent on older models) driver's side 10mm tie down bolt and hook
2) A 4 foot splice of stranded #12 yellow wire needs to be added to the negative battery lead, as the battery on the older models is on the driver's side of the trunk.
As for weight limits, I have the trailer hitch formerly sold by Coastal Tech on my 2002 Prius. It was recommended for a tongue weight not to exceed 200#, and a trailer weight not to exceed 1000#. I've used it successfully with a hitchriders.com dual trike rack that weighs at least 50#, plus another 50# each for 2 trikes, and will soon be using it with a 200# Harbor Freight trailer designed to pull a 600# boat, adapted to instead pull a 100# tandem trike.