In the end of the movie's climactic plot twist, how Fletch was to be killed by Stanwyk and then Stanwyk flees to Rio with the $3 million?
Stanwyk was going to shoot Fletch and then stage a car accident in which Fletch was burned beyond recognition in Stanwyck's car. The police would think it was Stanwyck in the car, since he and Fletch had the same build. Meanwhile Stanwyck would fly off to Rio.
Fletch realized this was the plan earlier...
Fletch: You know, what tipped it for me was something your wife said while we were in bed together.
Alan Stanwyk: Oh? And what was that?
Fletch: Curiously, she said we had roughly the same build. From the waist up, I imagine.
In fact, he wrote a letter to Stanwyck outlining Stanwyck's plan...
"Dear Mr. Boyd: Alan Stanwyk murdered me tonight. Charred remains found in the Jaguar are mine, not his. Mr. Stanwyk, using my name and passport..."