Hello,
I installed a Kenwood DNX8120 in my Z. It has the input for a rear view camera. I bought a rear view cam on ebay for $30.99 shipped and it works great. It has the standard yellow video plug and a 12vdc plug. But the neat thing about it is that it also has a +12vdc and Ground wire to give you more connection options. Heres the link.
New Waterproof Car Rear View Backup Camera w/ 5M Cable - eBay (item 320365007185 end time May-04-09 00:33:20 PDT)
I figured for 30 bucks if it breaks I'll just replace it with another one!
Take care,
Willie