[M. Lillis]

177 has a combination of written test on the rules and skill (Which is determined by going through training and then time trials). We rotate pairs during training and time trials to determine who works best together, then pick the best backup for each role. (Communication is key in chosing).
Experience also plays a huge role in the decision, because experienced drivers can usually handle the stress of gameplay better and know what to expect. (Also due to the fact that the coach knows what the driver is like after a few events). So this may be why you see a lot of teams using similar drivers over multiple years.
Hope this helps!