you have several options with aftermarket.
one is buy the pioneer avic 80 thats the newest one and seems to be best in my opinoin.
second is buy a screent preferably touch screen 7inch but 6 1/2 will do to save some money and just find a spot to mount the screen in your car that you can see good and dont get in the way of anything.
or second option is to buy a new head unit like i did.the pioneer 7500p or 6500p dvd lcd.and then just add the navigation too that and the screen is built in and there both touch screen.6500p is about 800 now and 7500p is about 1500.
hope this helps you somewhat :D