A rearview mirror is traditionally read as looking back while moving forward. It can stand for awareness of what follows you, or for a past that still takes up too much of your attention.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.