In regards to tracking the yellow car, Wilson absolutely got the information from Tom Buchanan. We find this out from Tom himself in Chapter IV right after Nick refuses to shake Tom's hand and asks what Tom said to Wilson that night:
"I told [Wilson] the truth," [Tom] said. "He came to the door while we were getting ready to leave, and when I sent down word that we weren't in he tried to force his way upstairs. He was crazy enough to kill me if I hadn't told him who owned the car. His hand was on a revolver in his pocket every minute he was in the house." (Fitzgerald 180)
Tom had it in for Gatsby, of course, for loving his wife (Daisy), . . . but also for what he thought was running "over Myrtle like you'd run over a dog" (180). Myrtle, of course, was Tom's mistress. The irony here is that Gatsby wasn't even driving the car. Daisy was.
In regards to what happened to Wilson, there is no doubt about that either. They find Gatsby shot to death in his own pool and make another stunning discovery as they begin to move the body back into the house:
It was after we started with Gatsby toward the house that the gardener saw Wilson's body a little way off in the grass, and the holocaust was complete. (163)
Wilson kills himself. Further, I absolutely love the way this scene is handled in the movie version of the novel, . . . with the white tulle curtains blowing in the wind. You never actually see what happened, but you know it just the same. Fitzgerald used a similar method within the novel. Fitzgerald: a tortured literary genius!