I believe the two mountains of brass indicate a time of judgment and the exact location refers to the valley between Mt. Zion and the Mt. of Olives. This valley is called the Valley of Jehoshaphat in Joel 3:2, 12. This is the valley of judgment as indicated in Joel 3:14 - "Multitudes, multitudes in the valley of decision: for the day of the LORD is near in the valley of decision." The multitude of Jewish graves on the eastern slope of the Mount of Olives express their desire to be close to the front of the line on judgment day, thinking this to be the time of greater leniency.
The identity of the "north country" (v.6) is difficult to determine exactly. Technically, if it is an individual country, it could be Syria, Iraq, or Russia. It might even be Turkey. However, it probably refers to a northern alliance during the tribulation. Daniel 11 speaks of the "king of the north" 7 times (v.6, 7, 8, 11, 13, 15, 40). It might be an alliance between countries like Syria, Iraq, Turkey, and Russia.
The "south country" always tends to point to Egypt. However, there may be other countries in alliance here as well.
Overall, I still am not settled as to the import of the passage. The brass usually refers to the time of judgment. The valley points to the Valley of Jehoshaphat and the judgment of the nations at the end of the tribulation. However, the exact purpose of the chariots still alludes me. Are they gathering the Gentiles? Or the Jews? Or, are they stirring things up for the battle of Armageddon so that the time of judgment will draw nigh? The bay horses going to and fro (v.7) connect this last night vision with the first one in Zechariah 1 where the horses among the myrtle trees are "they whom the LORD hath sent to walk to and fro through the earth." (1:10). They find the earth at rest (1:11). The time for God to restore Israel has come(1:12-17). The seems to be the meaning in Zechariah 6 as well. Anyway, the entire passage is fascinating.