Top left and right: At the beginning of the movie during the opening credits, the audience is shown two men transporting the woman who later assumes the name 'Rita', in a limousine on Mulholland Drive at night. Note the license plate number - '2GAT123' (you can click on the image to enlarge it). Above left: Near the end of the movie, Diane is being driven by one man in a limousine on Mulholland Drive at night. Diane is headed to the party at Adam's house. We note that the license plate number is the same as that of the car that carried Rita. Above right: We're shown the 'Mulholland Drive' sign during both of the driving scenes depicted above. The point is that the trip we're shown first is some sort of representation of the second one - in reality, only one car trip on Mulholland Drive takes place in the movie, that in which Diane is being driven. At some later point in time, Diane has a fantasy or dream in which she re-experiences the trip as a different woman. (The scenes are presented to the audience in reverse temporal order.) Ultimately, what's being suggested is that Rita is, in addition to being a representation of Camilla, some sort of representation of some part or aspect of Diane herself. (The blond wig Rita at one point puts on, is another suggestion of this representation, since Diane/Betty has blond hair.)
Recall that when Rita opens the blue box gathered at Club Silencio and then looks inside of it, she is 'sucked into' it, and then it falls onto the floor. We said earlier in the analysis that the box represents Diane's subconscious (and/or her past). Thus, the blue box scene is another indication that there's some relationship between Rita and Diane.
[UPDATE 10/28/11: If Rita represents some component or aspect of Diane's subconscious (i.e., her unconscious), then Betty's sexual union with Rita might represent an attempt by Diane to unite with her own unconscious, an act which would result in insanity.]
