I've installed a kemwood dnx 6140 model in my '03 nissan pick truck,like 2 years ago,everything went fine,until now when the navigation doesn't work,when i press on the feature"NAV",the screen gets all black,no maps,no directions .no nothing,it's kinda weired because all the other features such as dvd player,i phone,radio,they all work,just the navigation doesn't work,coul you please help me with this problem.Thanks