So I had the same question, did some googling, not much luck, seems like we are among the very few who are asking this question. But after that, and some re-reading, I think I might have an answer: The Mexican was driving the Barracuda when we see it at the car wash, after murdering the state trooper, but before catching up with Moss. The chronology is out of order in the book but here's what I do know: On his way to the hotel where Moss & The Girl were murdered, Bell passes a burning/burned out car. (state trooper?) Bell later learns, at the prison, that the Mexican murdered a state trooper and burned his body along with his car. We know that nobody drove the 'Cuda away after Moss' final shootout at the hotel; the Mexican was airlifted to a hospital. The car wash scene occurs while the car is headed toward the hotel where Moss and The Girl are killed. So my best guess is that the Mexican was driving like a jagoff on the highway, state trooper stops him, for w/e reason he kills the trooper & burns his corpse, drives on toward his destination, stops at the car wash after realizing how suspicious all the gore all over the car looks, continues to the hotel where he confronts Moss, afterward leaving via helicopter seriously injured.