The 73-year-old icon surprised viewers with a performance of "Waterloo" on Wednesday night's finals of "America's Got Talent."Decked out in form-fitting purple jumpsuit and blonde wig, the Oscar and Grammy winner brought the house down with the ABBA hit.It made sense given that Cher starred in the film "Mamma Mia: Here We Go Again." She released a CD of ABBA covers last year.Singer/pianist Kodi Lee, 22, was named the winner of Season 14.