To unlock each ending, you must finish the game a certain way. For ending I, you'll need to:
Optional: You can start at Bracciano (level 2 below)
Level 1 - Keep right.
Intermission 1 - Nothing.
Level 2 - Win or Lose the race, keeping right.
Intermission 2 - Leave Olga ("better to be apart").
Level 3 - Drive to the finish, not speaking to the priest at all (you can ask him where he is going but after few topics ask him "What" instead of "Why").
Finale - Nothing.
Video courtesy of Dernop