If you have the trailer hitch and the connector at the rear, you have four wires under the dash for the brake controller. From memory, I believe the bundle of four wires is just to the right of the steering column.
Dark Blue is the feed from the brake controller, back to the trailer brake system
Red is the hot from fuse 83 to feed the brake controller.
Light Blue is the brake switch input
Black is ground.
The wiring for the controller will most likely be different colors.
Tekonsha makes a great controller. Make sure you get a proportional controller.
As for the trailer. That will be pushing your H3 to the limit. I tow a 15" popup that can max out at 3200 lbs. I have pulled it with an estimated weight of around 2600 lbs. On flat area it is not a problem. Climbing any type of incline starts to run the engine in 2nd gear at close to 5K rpm. It pulls it, but it sure makes you wonder when it will blow up.:giggle: And a popup has very little air resistance when compared to a full size travel trailer.
With an auto, max is 4500 lbs, with 600 lbs max tongue carrying weight.
