I'm going to tell you how to beat the bomb collecting mission;
First, go into the maintenance tunnel, take the white car and collect bombs until Carlito tries to kill you. As soon as Carlito gets to you, you should be at the second bomb. Go through the doorway and go up the stairs into the first floor, then walk to the next maintenance tunnel entrance that is closest to the next bomb, retrieve the bomb then walk back up the stairs and repeat until you make it to the last one, then steal the truck beside the other truck that has the bomb in it and drive to the exit.
P.S. When you have time if you steal a car and go into the maintennance tunnel it is a very good way to kill zombies
Hope I helped.