The Nav computer is in the cubby-hole above the CD Autochanger. I'm guessing you've removed the seat bulkhead carpet in the boot? You will have had to overcome the cubby-hole door release cable. On the bulkhead in the boot to the left (as you look towards the front of the car) should be a metal bracket behind which is your bluetooth module if you have one. In the middle you should be able to see the back of the CD changer and nav computer.
HTH