I don't see a fuse for the backup lights in the wiring diagram. There is a backup lamp relay on one of the circuit boards inside the TIPM.
The first thing I would check is see if you can hear the relay click on and off as someone puts the transmission into and out of reverse. Use a mechanic's stethoscope or the screwdriver handle to the ear method.
Since you are getting no power on the wt/gy wire, trace it forward, probing for power every few feet. See if it has power before you get to the TIPM.
2008 Unlimited Rubicon Automatic