Annatto Oil: In a small saucepan, heat oil over low heat. Stir in annatto seeds, and cook for about 5 minutes, stirring occasionally. Cool, and strain it before use, removing the seeds.
In a large mortar, crush 24 cloves garlic with oregano and 3/4 cup salt. Stir in the sour orange juice. Set aside.
Make deep cuts in the pig on the neck, just under the lower jaw, on the loin, legs, shoulders, and ribs. Rub the seasoned orange juice into the cuts, as well as all over the skin, and in the cavity of the pig. Cover the pig with cheesecloth, and refrigerate overnight.
Build an open charcoal fire over a bed of stones. Hot coals may need to be added during the long cooking period. Place a post with a Y shape on either end of the fire.
Pass a smooth pole with no bark through the body of the pig. Tie the front legs tightly around the pole. Do the same with the hind legs, stretching them as far as possible. Resting the ends of the pole on the Y posts, place the pig over hot coals. Rotate the pole constantly and slowly in order to cook the pig evenly, and baste it frequently with the annatto oil. Roast the pig for about seven hours, or until a thermometer stuck into the thickest part of the thigh reaches 145 degrees F (63 degrees C). Carve.
Meanwhile, prepare the garlic sauce by first crushing the remaining 16 cloves garlic, the 16 peppercorns, and the fresh chilies in a mortar. Transfer mixture to a small bowl. Stir in olive oil, vinegar, lime juice, and 2 tablespoons salt. Serve with carved meat.