Back-up camera wiring

What type of connection does it have to the back of the head unit? If it's the typical RCA type connector like the image, you can buy extension cables, you would remove the head unit, disconnect it, then mount the camera in the rear and run the cable forward. Now where the cable stops you can determine how long of an extension you need. However you need to be careful of where the coupler will go. You don't want it in a spot where it gets damp or is subject to damage. So say the coupler would end up in the middle of the front door in the bottom trim panel. Move it back to the pillar and add some to the extension.
Then you get the extra cable and a coupler like the second image. Connect the cables together and run it to the head unit. Now before you install any trim or finalize the install test it to be sure it works. Then go to the coupler and wrap it with some electrical tape. Then coat it with some liquid tape. You want to seal it as much as possible so it doesn't get water in it. The cables and connectors are common items, places like best buy or similar will have them.

Not easily done as you would need to cut and splice the shielded video cable. Unless it has an easy connection at the camera itself I would add to the front where it already has the connection.