This is an entire walkthrough for the Reefer Madness case so warning, this does contain spoilers.
The case begins and you are given the assignment to go meet a snitch who tells you about Juan Garcia Cruz. He gives you an address and also tells you that Juan likes guns which gives you an indication of what to expect.
When you get to Juan's house a cutscene starts with him shooting at you. Stay low and move to your car where you can grab a better gun from the trunk. Go to the left of the house towards the back and jump the fence. Kick in the back door and head towards the front of the house. Juan will turn to shoot at you so kill him. He also has a buddy inside the house with him so kill him as well. Only then will it unlock the achievement.
Begin by searching Juan's pockets. You'll find his wallet with ID and a silver dollar with a some letters on it. To the left of his body on a box is a note with "Mastangkay" written on it. Walk forward to the stacked boxes to get the clue for Parnell's Soup Company. At this point you can choose to phone in for the address or just continue on.
You can also choose to go outside to talk briefly to a neighbour about Juan spending all of his time in the chicken coop out back or just walk directly to it. Kick open the door and on the shelves are some cans. There is one light coloured can on the second shelf. When you go to pick it up you find that you have to pull it instead which opens a secret door. Enter the room and go straight to the shelves in front of you by the can opener. Open the two cans using the can opener.
Go to the desk on your left and look at the ledger. Tap on a Tijuana name for the clue of "E.J". I believe you get the address for Parnell's Soup Company here if you haven't already rang in for the information.
Finally go to the back bedroom and on the table is the flyer. This will tell you about 20th Century Market. Go there now. If you've done everything correctly it will begin with a cutscene and once that's finished the achievement will unlock. Otherwise when you get there the shop will be closed.
After the cutscene you will chase the shop keeper to the roof where you fight him. Once you win the fight you bring him back down for questioning. You get his ID - Airto Sanchez.Note: There seems to be a few variations that can happen here as noted by bmlx5867. You can catch Airto before he gets to the roof which results in going directly to the raid after his interrogation. If this happens you can cancel this by responding to his first question about E.J. incorrectly. If you get to the roof but answer this question correctly you also go directly to the raid. On my playthrough I got to the roof and answered this question incorrectly.
- Knowledge of 'E.J.' - Lie
(Airto's silver dollar)
- Parnell's Soup shipments - DoubtNote: The game seems to want you to go to Parnell's Soup Company before going to 20th Century Market (and so missing an achievement) but if you do this on a different playthrough you get another question for Airto:
- Juan and Jorge Cruz - Truth
Go to Parnell's Soup Company where you talk to the owner, Howard Parnell. You ask for a list of employees. Search through the ledger to find Jorge Garcia Cruz.
- Factory sealed soup cans - Doubt
- Parnell's Soup Company - Doubt
- Knowledge of Cruz Brothers - Doubt
- Knowledge of Jorge Garcia Cruz - Truth
Parnell takes you on a tour of the factory and explains the various functions. He then brings you to factory worker Sergio Rojas who processes deliveries. Check the ledger on the desk next to him and tap on 20th Century Market.
- Factory sealed soup cans - Doubt
- 20th Century Market - Lie
(Sergio's silver dollar)
When you're finished here there will be a cutscene and it will be later on. You will now have to raid Parnell's Soup Company. There will be about 10 guys, maybe less, to take down so before you go in you can go to the trunk of your car to get a better weapon. Once you kill them all, you will be asked to follow a cop through the set of doors on the left to a small room. Search the body and find a coin in his left hand and his wallet in his front pocket. Go out of the office and open a box with reefer in it. Another cop will ask you about Jorge. Go upstairs and find his body with a lot of cash in his hand. His pocket will contain another coin.
A cutscene takes you back downstairs. You are now facing a table with five coins. Put them in the order of "MAS TANG KAY MET ALS".Note: BrumtownMangler made the comment of looking for a gap in the barbed wire fence out front to the left side of Mastangkay Metals and jumping through that to save time.
Head to Mastangkay Metals. You have to go around back to the alley. You will be able to climb a pipe to the roof of the adjacent building and then down another pipe to the grounds of Mastangkay Metals. Go to the back door and you will spot several bad guys at a table. Once they see you they will start shooting so kill them quickly. Head to the front of the building and up the stairs. There are more bad guys to kill along the way until you get to the main office to arrest the leader. That's it, case closed.