This mission for Madd Dogg will unlock once you complete The Meat Business for the Caligula set of missions.
REWARDS - Respect
It seems some rapper is drunk and threathening to jump off a ledge. Once the cutscene is over, run to the pickup truck with a blue marker overhead, then drive into the red marker. In the next section, you can only move the truck forwards and backwards (X and Square), so try to line up under Madd Dogg as he stumbles back and forth.
Eventually, Madd Dogg will jump and you have to catch him. Once he lands in the back of your truck, you'll need to drive him to the hospital, marked by the yellow blip on your radar. Drive CAREFULLY to the place; every bump and hit while in the truck will deplete his health a little bit. You don't have to worry about anyone, so get to the hospital and drive into the red marker to finish the mission.