EMPLOYER: Joe
PREREQUISITES: MM #44 (Minor Turbulence), Must be Trevor
REWARDS/UNLOCKS: Stun Gun
COMPLETION REQS:
1. Mariachi My Ride: Steal the band's Tornado after stunning them.
2. Stop the Music: Stop the band within 40 seconds.
After completing the Main Mission "Minor Turbulence" you can find a new Strangers and Freaks mission in west Sandy Shores. Head there to meet Joe and Josef, two law-abiding citizens on the hunt for illegals! During the scene they will essentially recruit Trevor to the cause.
Drive them to the bar in question (where you are banned - or should be...) and enter. No one of any illegal status is in here, so head back outside. Once you do a Mariachi Band will drive off. Jump in the van and follow!
You have to stop them quickly for one of the medal requirements, and the best way to do that is to shoot out their tires (after ramming them preferably). If you can't do that don't worry too much: just follow them through the desert and they will eventually crash. Once you have caught up with them they will get out of the car so do the same. You will get the Stun Gun here, so give them both a zap.
Once you've zapped them both, get in their ride for another one of the mission's medal requirements (if you wish), but the two band members will give up and get in the border patrol van. Your work is done and you can bet that isn't the last you'll hear of Joe and Josef.