From the looks of the manual on your HU, you have a rear video out and rear audio out. The rear video out is RCA and the audio looks to be a headphone jack style plug. They sell a splitter that goes from the headphone jack to RCA in on the rear display. You need to find out how the stock unit connected to the rear display.
Im not too familiar with the stock setup, but if you could drop the rear display without having to remove the headliner, I would see if you could identify the wiring that goes from the display unit to the front to get a better idea of what wiring was used to get signal and sound to the rear display.