I did this a while back and remember not finding any guides back then so I thought I'd show you how I did it.
Parts needed:
reverse camera and license plate. You can find these for cheap on aliexpress by searching for "rear view camera license Plate f20"
I recommend getting a 4 pin one. You will also need a 4 pin extension cable.
Steps:
Remove license plate light and replace it with the one that includes the camera.
Open the boot and remove the lower part of the rubber seal.
Either use a thin plastic trim tool to route the wire as far as you could to the right or do what I did and pull the wire through the gap and then pull all of it through(took a bit of time). You then reinsert it at the rightmost part of the gap.
After you drop the wire in, feed the rest of it in, it should hit the ground or at least get close to it.
Now get under the car and feel for the black part in the picture below. If you reach it from below you should feel the ventilation? flaps.
Feed the wire through one of the flaps.
Now that the wire is inside the vehicle the rest is easy.
