Dorian, the vivid and distressing imagery in this dream suggests a deep-seated anxiety or fear of loss. Watching a close cousin get mutilated in a car wreck and ultimately dying can be symbolic of your fear of losing someone dear to you, or it might represent a part of yourself that you feel is being 'wrecked' or 'destroyed' in some way. This cousin, being close to you, likely signifies a part of your inner circle or even an aspect of your own personality that you hold dear. The car wreck could symbolize a chaotic or uncontrollable situation in your life, something that you feel is spiraling out of control. The mutilation aspect indicates a sense of irreversible damage, something that can't be easily fixed or healed. This might be reflective of a situation or relationship in your waking life where you feel a sense of helplessness or inevitability.
Additionally, witnessing such a traumatic event might suggest that you are currently experiencing or anticipating a significant change or upheaval. Your subconscious mind could be expressing the pain and turmoil associated with this potential change through the graphic nature of the dream. The death in the dream signifies an end, but in the realm of dreams, death often points towards transformation and the beginning of something new. It might be your mind's way of processing the end of a current phase or relationship and preparing you for what comes next.