why are my rear tires wearing on the inside of my rear tires?

Hello, have you had the tires rotated at some point? It's true that the rear wheels on your 2005 Mustang are not independently adjustable, unless you have a high end Cobra perhaps. Otherwise, you have solid axles. If the tires have not been rotated (meaning wear started when in the front and continues now in the rear), you could have rear wheels that are not aligned properly -- but this is not an adjustable angle. A good alignment machine and technician will take 4 wheel measurements to see where the wheels are reading anyway, and if they are indeed improperly set, something is bent, or perhaps even the entire rear axle would have to be loosened and reset onto the frame. This is highly unusual though. Granted, the vehicle is 12 years old; physical damage from the past is definitely a possibility. If your tires are wearing out in 3 months, something will definitely show up when measuring the wheel alignment angles.