I believe the NAV is just a rebadged Kenwood unit. In the back of it there is a regular composite video input (yellow plug) as well as a molex connector for a reverse camera. For whatever reason Subaru never decided to tell/advertise this to the car owners (that I've seen anyway). But sure enough, they can be used. The video jack on the back can take just about any standard video feed set to a 'normal' (ie NTSC TV) resolution. Most DVD players / video game consoles have a composite video out. The N64 just happened to be lying around so I used it.
For the audio plugs I just used an adapter to feed the audio cables (Red & White) into a headphone jack plug. Its pictured above. Once that was done, i just plugged it into the AUX jack in the armrest. Since all this was temporary, it was really the easiest way to do it with things I already had. As it is a temporary setup, it was connected to a 1 plug inverter in the arm rest cubby.