According to the wiring diagrams I viewed, both systems show ending at ground point G402. Unforturnately it does not show where that is. I would start at the backup light switch and follow the ground from there. Do a check for power at the connector. It should have power on the feed line then see if you can follow the ground. You can do a continuity check on the ground wire. If it is bad there will be no continuity to ground.
Looking at the diagrams again I see that I missed something. The auto tranny has the NSS and the manual has the back up light switch. Doing the testing at the NSS would reveal the problem. Since both systems fail together it may be that ground.