ok what is the reverse wire for?

and do i really need it hooked up?

the reverse wire tells the avic unit you are backing up. you only NEED this wire connected if you hook up a backup camara. otherwise, no harm no foul to not hook it up. if you want to you can, though. just make sure the wire you tap goes to 12v when you shift into reverse and ONLY when you shift into reverse.

so it has nothing to do with the navigation ?

or accuracy of the navigation?