Mix all the ingredients together. Have more achiote paste and sour orange juice on hand, because you want the mixture to have the consistency of runny pancake batter or house paint. If you have them, don some gloves and smear this all over the pig's head. Save any marinade that remains and put everything in a lidded container in the fridge overnight.
The next morning, get your smoker to 200F. I prefer mesquite here, but use any wood you like. Remove the head from the marinade, reserving any extra. Set the head in the smoker as is, don't wipe it off. Smoke for 2 to 4 hours.
Wrap the pig's head in banana leaves or foil, or both. Before you seal the head up, add any remaining marinade. Continue to cook at 225F until the head falls apart, typically another 2 to 4 hours.
Pull all the meat off the head, peel the tongue and chop everything up for tacos. Discard any bits that freak you out; for me, that's the eyeballs and palate.
Serve the meat on corn tortillas with red onions soaked in lime juice and cilantro.
NOTE: You can buy shelf-stable sour orange juice, naranja agria, in Latin markets, or make a substitute by mixing the juice of 1 grapefruit, 1 lime and 2 oranges.