ronthehandymanga is correct. If you have a hitch reciever on your 2003 AV it is up to you to find the right drop size hitch and ball to meet your needs. The electrical connection should be by your receiver as a round plug but some owners moved it to the rear bumper for clearance and convenience. You can find old threads about how to do that mod on this website.
If you are referring to the adapter plug that goes in the round plug to convert the wiring to a flat 4 pin wiring setup then you AV should have come with it and may be found in the glovebox or center console. If not it may have been kept by a previous owner. You can find those adapters at many trailer supply stores. You could also consider buying a T-adapter harness that plugs into the back of the factory plug and gives you a 4 pin connector so you don't need to use the round plug at all...I believe the part number of the harness is 18384 and here is a photo.