On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Arms Dealing. You can begin this quest in The Highlands during the completion of the main mission named Bright Lights, Flying City (the successful defence of the Beacon in The Highlands is a requirement for this quest).
The Highlands - northern part
1) Bounty Board
2) Mailbox 1
The Highlands - southern part
3) Mailbox 2
4) Mailbox 3
5) Mailbox 4
6) Mailbox 5
You can receive this mission from the Bounty Board  in the northern part of Highlands. Pick up the first package  from the mailbox (screenshot) in the camp nearby the board and the timer will start. You will have two minutes to collect all the packages, each giving you some additional times. You should consider visiting the spots beforehand and clear the area. You can also equip a Relic which improves vehicle Boost.
Beside mailbox 3  you will have to jump below and then head up along rocks on the right side of the building to reach the mailbox. In order to get out of there, jump onto the crate beside the building shown in the above screenshot.
Beside mailbox 4 , exploding Robots will keep coming from behind the door.
The last mailbox (number 5)  will be protected by Threshers who you should just ignore and climb up the ladder from the above screenshot. The last package can be found above.
Now return to the vehicle and quickly drive to the Bounty Board  (the timer is still on). Beside the Board you should find a mailbox (screenshot) into which you have to put the packages. If you make it in time, you will be able to choose one of two rewards.