the really big black wire with the metal end is for the antenna. the one that plugs to the out side of the unit is another ground

 

The black/green in your pic is the factory ground wire. Some Dakota's from the 2000 model year had a braided ground strap, some had a ground wire like yours does. In 2001 all Dakota's began using a regular ground wire like you have now. In either case it is best not to use the factory ground wire, especially with today's higher powered decks. The large black cable with the Motorola style plug on it is the antenna.

With all those cut wires and exposed wire ends I see in the background of the pic, I would say it would be a good idea to check your constant and accessory 12 volt leads with a volt meter to be sure you don't have any blown fuses. When you install the new deck, be sure to tape up or cap off any unused wires, like the factory illumination wires.

Jimmy