Hanging it from your mirror. Maybe.
Keeping a mask hanging from your rearview mirror can be a convenient way to always make sure you have one with you. Since you aren’t putting it on a surface, there’s no concern about contaminating a surface. However, you should either wash every mask between uses or put a worn mask aside for a couple of days before using it again, to allow any virus to die. If you want to leave your mask hanging from your rearview mirror, let a couple of days go by before putting it back on.