Put all the marinade ingredients in a saucepan and bring them just to the boil. Remove from the heat and cover the pan. Let the ingredients steep until the marinade has cooled completely. Place the meat in a large bowl, then pour the marinade over. If it's not enough to cover the meat completely, top it up with some more wine and vinegar. Let the meat marinate in the fridge overnight.
The next day, take the bowl out of the fridge and let the meat return to room temperature. Fish out the meat chunks and pat them dry.
Now strain the marinade liquid into the pan, leaving all the solids behind. There should be enough liquid to almost, but not quite, cover the meat chunks. Cover the pan and simmer for a good 60-90 minutes, until the meat is perfectly tender. Add any extra marinade, or a bit of water or wine, from time to time if things are getting a bit dry.
When the meat is just about done, add the finishing ingredients to the pan. Stir gently and simmer uncovered, until the chocolate melts completely and the sauce has reduced to your liking. Taste and adjust for seasoning: a bit more sugar if you find the sauce too sour, a bit more vinegar if too sweet, and a pinch of salt if it needs it.
Serve hot, with slices of toasted bread or polenta.
Notes
In addition to the total time indicated, you will need to marinate the boar meat overnight.