If installing only one unit, the best location would be in the middle of the roof (or as close as possible).
If installing two, it is recommended to put them diagonally from each other, in opposite corners of the roof. Often the preferred location is the front right corner, and back left (20in off the partition).
Our vents fit between the ribs on Sprinters and Transits, but become narrow when dealing with new ProMaster models.