My station wagon back up light wont work. checked fuse ok.

My station wagon back up lights wont work. Checked back up light fuse, ok. Removed tail gate inside panel to access back up lights. both bulbs good. Checked both bulb sockets for 12vdc power with car ignition on & shift in reverse. No DC voltage. I checked the wire continuity between both bulb sockets and there good. The socket wire to chasis ground is also good. I even went further and applied 12vdc to the interconnecting back up wire on the tailgate and both lights works fine. I am unable to check the continuity between the tailgate backup light wiring and the fuse panel without proper wiring diagram. Can anyone give me some help in troubleshooting this problem. More so in locating the different areas of the wiring so i can isolate the problem. Thanks