We are using Gretchen Wilson's song "I'd Love to be Your Last". Its so perfect for second (or beyond) weddings because it is about partners that have each had a life and made relationships mistakes or been knocked around by love. Its a beautiful songs as well since the music is very understated and doesn't overpower the great lyrics and her voice.
http://www.youtube.com/watch?v=MPAKgBcmRWM