With the Mariachis out of the car, approach them as Trevor. Be sure that you have the Stun Gun drawn. If you shoot and kill the Mariachis, you will fail the mission.
Stun both of them. You can get in 4 zaps before Joe tells him that they can handle it from there.
If you want to finish things properly, get into the Tornado that the Mariachis were driving and head ahead from the area in it.