I have 2005 Ford E350 How do i hookup trailer

If there's no pigtail or plug for trailer lighting, you can purchase an in-line connector that will plug into the existing taillight connection of your vehicle. Uhaul sells them as well as retail auto parts stores and trailer supply houses.
Simply unplug the stock connection at the back of the car, and insert the connector between the two and you've patched into the wiring.

etrailer.com has them as well.

Get a trailer light adapter from Walmart or your local auto parts store and follow the instructions in the package to connect it to the lights on your vehicle.