This mod replaces Carl Johnson's legs and texture for a nude version that fits your weight and muscle. According to your exercises, your "erect friend" will grow or diminish with lack of exercise. Be it Fitness or not.
Place the folder "Nude CJ BBC" inside
the Modloader folder (Downloaded at:
http://www.gtagarage.com/mods/show.php?id=25377). Do not waste your time
replacing files "gta3.img", this will give you an unnecessary job.
Good fun.
Download: