CinemaRetro has an excellent post on the making of THE FRENCH CONNECTION II, which took 
place in France. In the midst of production, director John Frankenheimer called reporter/novelist Pete Hamill to do an emergency rewrite on the script:
try to get there within two days. "Why not one?" he said, and laughed
nervously. I never asked why he called me. Someone hand-delivered a script to
my place in New York and I read it on the plane. John, at that
time, had a major problem. He had already shot nine days of the existing script.
He had developed a reputation for going over budget, so had no flexibility. He
couldn't re-shoot what was already in the can. That gave me a
problem too, since I had to write around the existing pieces, which, as always,
had been shot out of order. It was like working on a jigsaw puzzle.
It's not a very good movie, certainly not compared to THE FRENCH CONNECTION. Years ago, I read the FRENCH CONNECTION II novelization, which had a very different ending than the movie.
SPOILER ALERT.
In the movie, Popeye Doyle runs along the Marseilles waterfront, trying to keep up with the bad guy, who was escaping on a yacht. Popeye eventually gets a clear shot and shoots the bad guy on the boat.
In the novelization, if I recall correctly, there's a convoluted ending involving a powerline and the bad guy getting electrocuted in a bus.
There are some good things in the film. Gene Hackman’s performance, and Claude Renoir’s photography are both excellent. That Hamill story explains a lot about the story, though.
I’m glad I wasn’t the only one who didn’t like this movie. The pace was pokey until about 2/3rds the way through. Watching was just an extremely tedious experience.
I’ve never seen French Connection II, but it certainly sounds pointless. The messy ending of the first one was one of the aspects that made it a great story. There was no need for a sequel to tie up the loose ends.