Top left and right: At the beginning of the movie during the opening credits, the audience is shown the woman who later assumes the name 'Rita', travelling 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 in a limousine on Mulholland Drive at night. Diane is headed to the party at Adam's house. The license plate number of this limousine 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 a 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 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, a representation of some part or aspect of Diane herself.
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 part 4 that the box represents Diane's subconscious (i.e., her unconscious), and/or her past. Thus, this blue box scene is another indication that there's some relationship between Rita and Diane.
The blue box lies on the floor, after Rita seems to have disappeared inside of it.
