On this page of our game guide to Greedfall, you will find a detailed description of Dirty Traffic side quest.
Where to start the quest?: After completing the Contraband side quest.
Go and talk to Admiral Cabral. Then head to San Matheus and explore the barracks in the city - enter marked rooms and talk to all characters. Then go to the meeting place and travel to Tir Dob to ask Derde for help. You can use your intuition talent or try to rely on Siora to convince a woman (if Siora is on your team).
Now go to the sawmill in San Matheus, where prisoners are being held. Defeat the guards and free the prisoners. The key to the cell can be found in the corpse of one of the guards. You're gonna have to set a trap for Egon's soldiers now. Hide behind the crates and wait patiently.
When enemies appear, come out of hiding and defeat your opponents. Then go to meet Lieutenant Alexander and return to Manfred in New Serene to complete the mission. You will get + 2 reputations with Coin Guards and 1900 XP. You will also start the Egon's Face side mission.