David Lynch's last clue is "Where is Aunt Ruth?" It's the last clue for a reason: it's the most important of his clues in understanding the fundamental nature of Mulholland Drive. Where is Aunt Ruth? In a sense, she is everywhere. First we see who we believe to be Aunt Ruth at 1612 Havenhurst — a well-dressed red-headed lady with a scarf on, a taxi driver helping her with her bags. Then look at the airport while Betty is saying goodbye to the old couple. Who walks by around the taxi zone? A well-dressed, red-headed lady with a scarf on, and a man accompanying her carrying her bags. And then at Sierra Bonita, Betty and Rita watch a well-dressed red-headed lady with what appears to be a scarf around her, with a man carrying her bags to a limousine.
But where else is Aunt Ruth? I'm just going to flat out tell you: Hell. Let me explain.
In the dictionary, the word "ruth" is described as "grief; repentance; regret." This is key. I was always a believer that Diane was abused as a child, most likely by a family member — not only because of the clues in the film, but also because of the prevalence of child abuse in Lynch's work. However, I was never really convinced that her abuse was central to the film's meaning, until now. Diane's uncle or father sexually abused her as a child, and Aunt Ruth knew about it. Diane threatens to tell someone what's going on, but Aunt Ruth couldn't bear the secret to be let out. So she makes an agreement: if Diane keeps quiet, she can have some or all of her aunt's money when she's gone. Silence. There are those who believe one of the greatest sins there is is silence. Diane's agreement to be silenced is not just for the money — the primary reason is that Aunt Ruth has put things in her mind to make her feel guilty about it.
The constant appearance of an Aunt Ruth-like character — always packed and on the move in her dream — could represent either Ruth's unwillingness to help Diane in her time of need, or Diane's unwillingness to accept that they had an understanding over such a traumatic thing. Coco says "You and your Aunt probably have an understanding, so here's the key." Diane is given the key to Aunt Ruth's stuff because Aunt Ruth is dead, just as Diane is given another key because Camilla is dead. This also answers one of Lynch's 10 clues: who gives a key, and why?
Why is Aunt Ruth in hell? Look at the flyer on the lamppost where Betty and Rita get into the black cab to go to Club Silencio. The only readable word on the flyer is in block letters: "HELL." I believe Diane gives herself a glimpse of Hell, or what she perceives Hell to be. On the parking lot of Club Silencio is a figure 8. Turn it sideways and it's the symbol for infinity — showing that we are now entering a realm of eternity.
After Betty and Rita are seated in the club, the Magician starts talking. I believe the Magician is the Devil, or Diane's perception of the Devil — a pretty silly Hollywood stereotype, just as the Cowboy is a silly stereotype. The Magician speaks at least three other languages. The Devil's many tongues is a frequent description of him. He has a goatee, like most standard depictions of him. He has a large wand that could act as his pitchfork. He can perform magic. And his grin as he disappears behind smoke doesn't exactly say "Nice to see you."
When the Magician says "Listen," thunderous sound fills the room, and blue light fills the theatre — symbolising her abuse, just as it does in Fire Walk With Me. This is Diane's idea of silence being broken, and she quakes with fear, but afterwards appears relieved, as if a weight has been lifted.
The reason for the Magician speaking English, Spanish, and French? He is speaking to three people: Betty, Rita, and Aunt Ruth. Aunt Ruth is associated with French because inside her apartment is clearly seen the book Tout Paris — a guide to the art of French decoration.
Next: the Blue-Haired Lady is Aunt Ruth. The club is almost all red, just like her hair, but you'll notice it is sometimes flooded with strange blue light. As Aunt Ruth has the only balcony seat, she holds a special place in the club, and her hair has turned from red to blue — she has been tainted with the guilt of silence. Where is her scarf? I think the scarf may be a symbol of her pride which forces silence upon Diane, and when she is sent to Hell she is stripped of that pride and forced to feel how wrong she was.
Finally, the film ends with the word "Silencio" — a reminder to both Aunt Ruth and Diane that the abuse and subsequent silence has, indeed, been "the end of everything."