Which part?
I'm guessing you meant mike because if you missed anything it might not all make sense. In the Season finale last year mike was going to kill hector Salamanca. Before he could do so someone got to his car, left a note on his windshield that said "don't" and stuck a branch on his car horn so he would hear.
Mike finds a tracking device in the gas cap of his car left by whoever stuck the note on his windshield that didn't want him to kill Salamanca. He talks to a former coworker(I guess) and gets a tracker that can locate the tracking device from his gas cap. He runs the battery down on the tracking device he found in his gas cap so that whoever put it there will have to pick it up to change out the battery. When they come by the house to pick it up he turns on his device locator and follows them to find out who it is.