Put wooden skewers in a pan of water for 30 minutes, so they don't burn on the grill.
Heat the oil in a small saucepan over medium-high heat. Add the achiote seeds. Turn off the heat and let the seeds steep until the oil turns a vibrant orange-red, about 5 minutes. Strain the oil through a fine-mesh sieve into a blender: discard the seeds. Add the vinegar, garlic, aji amarillo, cumin, salt, and pepper. Cover the blender and puree until well combined.
Place the pork in a resealable gallon-sized plastic bag. Add the achiote marinade, turn the bag to coat the pork, squeeze out the air, and seal. Refrigerate for at least 4 hours or up to overnight.
Preheat an outdoor grill or grill pan to medium-high (you may also preheat the oven broiler.)
Remove the pork from the marinade; discard the marinade. Thread the pork onto the skewers alternating with the pineapple and red onions. Grill (or broil) the pork skewers until they are browned on all sides, 4 to 6 minutes total cooking time. Take care not to overcook or the pork will become tough and dry. Serve the pork skewers on a bed of lettuce leaves and garnish with lime wedges. Pull the pork off the skewers onto a lettuce leaf and eat like a taco. For a great presentation, stick the skewers into a whole pineapple and put on the table with the lettuce and lime on the side.
Recipe courtesy of Ingrid Hoffmann, 2008