After completing the Hard Talk quest head out in Mowen's boat. Talk to Mowen when you get to the city. When you get to the waypoint, go around the fence and climb the ladder on the cargo container to get inside the red one on top. Get Mowen's package. Head to the gas station load up two fuel canisters into a truck then drive as close as you can to Mowen's boat. Then take the canisters one by one to the spot marked on your map. Talk to Mowen to complete the quest and start the Back in Black quest.
Phoenix Mod - Inside the red cargo container on the shelf next to Mowen's package.