https://youtu.be/snG1vnkoRZ0
The link take you to a video of Brent Smith explaining exactly what "Black Cadillac" is about. He talks of respecting death, how "long black cadillac" is in reference of a hearse, and explains how the song is essentially a 'warning song' about living everyday like it's your last and if you "eff up too many times you might be the one waving bye in that cadillac" metaphorically, death.