When you receive mario kart 7 data from someone via streetpass, go to the Mario Kart Channel to access the streetpass data which includes the kart customization that person set on his channel profile. Select Grand Prix to race in a Prix that person set up on his profile. Place ahead of that person's mii character and if that person choose a kart customization that includes a part that you don't have yet, you will unlock that part. For example, if the person you street passed used a kart body you didn't unlock yet and you beat his/her mii in his Grand Prix, you will unlock that kart body for your own use.