Assuming the priest’s level of belief is paramount, does the addition of holy water to the exercise make any genuine difference?

In the case of the blessing of an object or a person by the priest, the addition of holy water can make a material contribution for reasons we have discussed with you. If the priest blessing a vehicle also blesses the holy water in use, this will add little to the overall exercise other than the act of using holy water through conditioning adds somewhat to the belief quotient of the priest because they are not acting on their energies alone but the energy of the divine holy water, and so will be more confident of the overall benefit. So the two go together and have a kind of synergistic effect to confer a positive outcome that a person alone might not be able to generate.