Mariah Carey and Willie Nelson have become music's oddest couple after teaming up to write a new movie tune together. Grizzled country star Nelson is co-writing a new song with Carey for her next movie TENNESSEE, in which she plays an aspiring songwriter. Carey will also pay tribute to her new songwriting partner in the movie, by singing Nelson's ballad HELP ME MAKE IT THROUGH THE NIGHT. Tennessee started shooting in New Mexico last month (JAN07).