What needs to done depends on what kit is being fitted, what model, but more specifically which type of rear light is fitted.
I will specifically cover 2015 year as this is what is being discussed.
So you have two styles of Kit
68474-04B which moves the indicator back on the strut.
Kits 67800071/7 which moves the indicators to a bar on the rear number plate mount.
You have 3 main type of rear lights
The standard rear light where the indicators terminate on the circuit board within the light.
The custom style LED light where the indicators terminate under the seat.
No separate rear light as such where the rear/stop light is combined with the indicator. (These kits are not for these models)
So Kit 68474-04B will require the wiring to indicators to be extended unless you were fitting it to a vehicle with the indicators already fitted to the rear numberplate, so 1200T, they then may well be long enough as the terminate under the seat!
With the standard rear light the wiring is simply not long enough to thread through to the adaptor plates and reach the circuit board.
With the custom LED style light you will again need longer wiring, as above to thread through the adapter plates to reach the connectors under the seat.
(Connectors 18, 19 & 40 are under the seat)
With Kits 67800071/7 due to having to feed the wires through the indicator bar and to exit the left hand side, with the Standard type rear light the wiring will certainly not long enough for the R/H indicator and unlikely to reach with the left.
With the custom LED light you are moving the indicators further away from the connectors under the seat and also have to again feed it through the light bar, so again will need to extend the wiring.