Did the camera come with the harness so you can plug the camera to the back of your HU? If you have the harness, the PINs should match one of the connections to the back of you HU. Looking at the pic of the rearview camera, I only noticed the red and yellow cable.
I have been asking the same thing for years. You will need to find someone who can build a custom interface between the navi unit and the RCA cables.
Also see pictures below how connect DVD player to the HU in the Mazda CX-7. I want to make similarly, but with the camera! But in my HU do not preset such Connector pinout as in CX-7 HU (see IMG_13).
Definitely try to find the wiring diagram for your HU. I don't know which PINs you should use. Comparing img_10 and img_2_n, I would try to use the wider connection that is free in your HU. The connections on your HU and the HU in the CX-7 look similar to me.
If you can't find the w/d, I would try and disassemble your HU and check for board labels.