This is an unmissable storyline achievement.
When you wake up at Mo's head to the trailer towards the left of the map. When you get there the trailer is locked and the owner, Todd, is downstairs in his basement.
You need to get into the basement to retrieve Mo's welding torch. In order to do this you need to knock on the door, which brings Todd up.
Here is also where you can receive the 'Angels with Dirty Faces' Achievement, simply keep talking to Todd until this unlocks after Ben offers to sell him some fine leather jackets.
Then, while Todd is still looking out through the peephole, kick the door down, which will knock Todd out. This will unlock you the 'Trailer Trashed' Achievement.
When you enter the trailer, there is now an exposed lift/elevator down to the basement, and here is where you will find Mo's welding torch.