To access Mexico early, travel to the bridge called "Frontera Bridge." Ride your horse quickly to the end of the bridge and get off the horse, letting it plummet into the water below. Do not call a new horse. Walk backwards until the railroad tracks start running into solid ground, turn left and go down a steep slope to the riverbank. There should be many small bushes by the river here. The music should also switch. If you don't hear the music, start over. Set up a campsite when you hear the music and you will find yourself on the Mexican side of the river. To get back, you have to travel to the bridge called "Romita de la Baya." Do the same thing with the horse here, let it fall into the river and set up a campsite on the island. The campsite should be near the edge of the island to the north, either to the left or the right of the bridge. You can also listen to the music here, when you hear the music change back to the "American" music, you can set up your camp and you will be back on the north side of the river.