The partner has heap of variations in body styles, engines and transmissions. You also don't tell us why you're trying to even find it.
This wire isn't hard to determine, but without understanding why you're after it, here are some suggestions.
Perhaps a complex way is to open the area where the reverse light is, and take note of the wiring colour, and then attempt to trace / find it in the front cluster. If taking this approach, I would attach a multimeter and probe the corresponding colour wires in the front looms to find it again.
Another way is to find the reverse sensor on the transmission or for either of the above, simply just run new wire you spliced into the system before it disappears into the wiring looms. If for example, this was for reversing camera, they normally have a red wire that is integrated with the video cable for splicing into the rear reverse light on the positive side, and you have to run video cable back anyway so just splice it in the existing reverse light.
If you provide additional information, I can look up the wiring looms and give you general where to find it, but not even sure why you're trying to find it or if going about this the right way.
Hope you find this helpful.