In a blender or food processor, combine the garlic, vinegar, juices, salt, and cumin. Pulse until well combined. Crumble in the achiote and pulse to combine into a very soft paste; add more vinegar by the tablespoon to loosen it up if necessary. Blend in the cilantro.
Cut the meat into lenghtwise slices 1/2 to 3/4 inch thick. Lay the meat between two sheets of plastic wrap and pound to an even thickness of 1/4 inch. Coat the slices generously with the paste and refrigerate for at least 6 hours and no more than 12 hours.
Half an hour before cooking, clrean the grill thoroughly with a brush and wipe it down with an oil-dampened rag. Preheat the grill on high.
Remove the meat from the marinade and shake off as much of the paste as possible. Place the meat on the grill, close the lid, and grill for 5 to 7 minutes on each side, checking often to make sure the meat doesn't burn or cook too quickly; if it does, move it to a cooler part of the grill. The meat is done perfectly when it has an internal temperature of 145 degrees. Do not overcook or the meat will dry out. (I used my George Foreman grill and just made sure to wipe it with a paper towel between batches) Set the meat aside, loosely covered with foil.
To serve, split the rolls and toast on the grill, then layer from the bottom up: cheese, meat, lettuce, tomatoes, guacamole, onion, jalapenos, and salsa. Skewer with a couple of toothpicks and cut in half.