1st, watch this video:
I've been driving a lot there and that must be the airport !
Okay, you know where is North Miami ? That's one of the biggest parts of Miami.
I tried everything to go there but it works only if you go true the airport.
So first go to the airport of Miami, then let you fall in that "Nothing" and hop your car will land on its wheels (and dont forgett to activate the Invincibility cheat). once you'be done that you go to North Miami (map). Your car will fall in the water and you'll automatically jump out. Then you will have to search after a open way to come at the surface. If you cant find one just go to the street and you should appear at the top of a building. Then jump down and umâ€¦ welcome to North Miami ! Have fun and don't forgett that you can fall true the city at any moment so be carefull !