How hard is it to install Nav/rear view Cam?

What unit into what vehicle?

I installed the Rosen nav unit into a Prius II. Very simple, only four plastic panels to remove, swap the units, no wires to splice or trace...All told, should take less than an hour. At the time I did the swap I didn't have a cable long enough to get the GPS antenna on the roof so I left it under the dash. It didn't get great reception under there so when I did the backup camera, I also bought an extension cable and ran the GPS antenna to the roof. Much better reception now.

The backup camera is a bit more involved. You must remove quite a few interior panels and route wires from the tailgate and the taillight up the side of the vehicle and over to the Rosen unit. Popping out the panels can be a bit unnerving and takes some practice but there is a good PDF from Toyota posted in one of these threads that describes most of what you need to do. I suspect if you have never done something like this before, you could easily burn 4+ hours trying to install a rear camera.

Read all the threads on the subject, even for different head units as most problems have been discussed and its really nice to know what to expect.