Taking in the Trash
Follow the waypoint to find a Trashmaster parked in an alleyway with a group of three sanitation guys standing around. Objective here is to drive the Trashmaster to wherever Luca tells you to go, picking up garbage bags filled with trash and "ice."
The first diamond pickup is in Chinatown. Stop at the waypoint at the side of the street. Luca's boys will get off and pick up bags and carry them to the garbage truck. Be sure to open the door to the truck's compactor by pressing button the little tutorial tells you to press. Luca's boys will toss the bags in and get back onto their position on the back of the truck.
The second diamond pickup is on H Street. Stop at the waypoint and wait for Luca's boys to pick up the bags full of ice and toss them into the back of the truck. After the last guy gets on the back of the truck, a cutscene is triggered.
It seems somebody has caught wind on your activity and has sent a car with armed occupants after you. Drive to Fishmarket South to drop off your load. Luca's boys will shoot back at the pursuing car once it starts shooting at you. Shoot back at the car since the Trashmaster is rather slow, mentioned by Niko and Luca during the getaway. Don't worry about getting attention from the LCPD because when you make it to the waypoint on the map, the heat dies down anyways.