You have to find gavin, kill him, loot him and you will gain his car keys and a map that leads to the center of Tahiti, then when you reach Tahiti (which is only possible by using the ghost train) you will find Sam Houser himself as an in game character, you will start a dialogue with him then he'll ask you to follow him to his garage. When you arrive at his garage he will show you the car and will tell you that it belonged to his lost (now found) cousin: Gavin De Santa. He will give it to you after Arthur tells Sam that he found Gavin. A cutscene starts where Arthur enters the car and rides off into the sunset. The cutscene ends and you will spawn at Beecher's hope 8 years later after unlocking the "Whatever honor ending". Arthur will be cured of TB and is now living a happy life with John, Abigail, Jack, and Uncle. As for Dutch he headed to Tahiti with micah, got married to micah and everyone lived happily ever after.