You can think of the opponent's grid like this:
1 4 7 10
2 5 8 11
3 6 9 12
This is the order they filled the territory. In other words, square 1 would be the first car the entered, and in this example square 12 would be the most recent to enter. When you remove an opponent, they shift up in columns and then left in rows. Essentially they maintain their order of entry.
You can use this to your advantage when attacking. If an opponent recently took a square, then the highest numbered square would be the most likely to still be damaged from their fight to get in. This is most often the ideal choice of opponents. However, if the opponent has held positions in that territory for a while, attacking square 1 may be ideal since that player has been there the longest and is therefore the least likely to still be online. And if you attack square 1, it's easy to have someone follow you to finish them off, as the opponent in square 1 will remain in square 1 until he's dead or replaced.
Note that the order of your own gang's cars in your defense grid is NOT what your opponent sees in their attack grid. That throws a lot of people off.