Fuzzy dice hanging from the rear view mirror became popular after WWII, when fighter pilots brought the tradition of carrying dice for good luck, back to America. It caught on quickly and is still a sign of an enthusiastic car person who loves the classics. Classics like the 1966 Mustang.
The 66 Mustang was popular and beautiful right down to the last detail, such as the rearview mirror with its iconic day/night turn switch. A car this beautiful gets cared for which is why it's still rolling since 66'.
If you wash your shirt inside out it will last a long time too and keep you rolling in style.