MY14 towbar wiring can be spliced in pretty easily. Here's some pictures and information to assist you get the job started.
Trim removal is fairly normal going. At the least you will need the left side boot interior trim off. Refer to this link for general trim removal guidance if it doesn't come naturally:
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STOP, TAIL, LEFT AND EARTH
Source was located at the base of the left D pillar in the boot. There's a large grey connector which can be unclipped from the body to provide a bit of working length. Note the trailer earth connection on the amplifier bracket bolt.
Source was located just around corner from the tail light behind the bumper and was chosen entirely due to the elevation of the exterior splice. I used a pick tool to push a hole through the ventilation flap's outer frame and ran the wiring back into the boot behind the sub. You could also drill and grommet behind the tail light itself, however it is double skinned here and can be a PITA as a result. Anyway, once inside run behind the spare tyre shroud to the previously photographed location.
RIGHT INDICATOR (alternative)
Source is under the vehicle towards the right side of the boot, and where you will probably have to source the right indicator if you don't want to remove the bumper, or drill behind the tail light... or simply think my method was obsessive or just crap.
Source was located under the sub, on top of the right back wheel in the boot. Not many people bother wiring this in, but there it is if you want to do it by the book. Run across the inside rear of the vehicle with the right indicator.
PLUG = LAMP = CAR
Red = Stop = White/Brown
Yellow = Left = White/Green
Brown = Tail = White/Grey
Green = Right = White/Yellow
Black = Reverse = White/Lime
White = Earth = Bodywork
Once all wires are in the left rear corner, you can choose one of several methods to route them out. As I still had the bumper off I punched a hole in the grommet located on the lower left corner of the car and then ran along the back of the towbar. Apologies... I forgot to take a picture. You could also go through the floor, which might be best if you tapped the right indicator underneath as per the "alternative" method.
PIN ORIENTATION (flat 7 pin)
Courtesy Narva... they also have all other standards detailed here:
There's obviously several methods you could adopt to run in trailer wiring. I'm not saying that this is in any way the best, it's simply how I did mine.