I got a Curt Class III hitch for my 2003 Sienna.
A bunch of the hitch manufacturer's are all owned by 1 of them.
A class III has a 2" receiver and is the more heavy duty version.....very close in price to the lighter capacity Class II that has the smaller receiver.
I had a local hitch shop order and install the unit and the wiring for the lights.
They can also take care of the brake controller.
A good hitch shop can guide you through the process and do a good installation.
My 2003 did not come with the towing package.
The towing package comes with a auxillary motor oil cooler that is at the oil filter location.
Coolant flows through a small "sandwich" heat exchanger at the base of the oil filter.
The towing package also comes with an auxillary transmission fluid cooler.
The location of the factory (towing package) auxillary transmission fluid cooler is inside the front, driver's side, fender.....ahead of the front tire.
There will be lines going there from the transmission and the standard transmission cooler that is mounted in the base of the radiator.
I installed a auxillary transmission cooler in front of the radiator.
Link to the first in a series of pictures that show this.
http://rides.webshots.com/photo/2417...11220610HeZCdx
Dual electric fans are standard on all 1st generation Sienna.
HOWEVER, towing package or no towing package may determine what fans are used and it DOES change the radiator cooling fan "logic" (when the computer turns the fans on and off).
In stop and go traffic, I keep the A/C on (defrost will do the same) as this keeps the radiator fans ON more, which will keep your transmission fluid much cooler.
I installed Airlift Air Bags inside the rear coil springs.
These do NOT increase the rated towing capacity of the suspension or vehicle.
They DO help to maintain the correct height of the rear of the vehicle under different loading conditions.
If you measure the normal height of the hitch from the ground without the trailer and luggage in the vehicle.......then measure the height once loaded, you then add air to the air bags (up to the maximum of 35 lbs) to restore the vehicle height to the normal level.
This helps the vehicle handle and ride better when loaded.
You don't need a air compressor for them (although they sell one) a simple bicycle pump is all that is needed to adjust the air pressure.
If you click on this picture, it will take you to the first in a series of pictures that show the installation of the bags and the routing of the air tubing.
I have the air tubing go up into the jack compartment.....and the bicycle pump connects to a tire type of air valve at that location to adjust the air pressure (routing and location shown in my pictures).
When figuring your towing capacity, there is a tongue weight.....the weight that is pressing DOWN on the hitch, and a trailer weight......the weight of the trailer INCLUDING anything in it.
THEN there is the total weight......the weight of the trailer (including everything in it) your luggage inside (and on) your vehicle......including all the people inside the vehicle.
Read up, learn......ask the shop that installs your hitch for pointers on towing.....well in advance of leaving for your trip.