I sent mine back only last week for a failed rear facing camera. Got to say the service was incredibly fast considering it whisps off to BRNO in the Czech Republic and back again!
But you have sod all chance of getting a plastic casing on the back, more likely to get a newly refurbished unit if the screen is knackered. You might be offered one of the GPS units if you did not apply for one at the time/and or and extension to your warranty. I took both, but found I barley use the GPS anyhow and the actual unit is sat in the car somewhere having only been used once! The only bug bare is that the weather widget struggles to locate me exactly.
And God said: DELETE lines One to Aleph. LOAD. RUN. And the Universe ceased to exist.
Then he pondered for a few aeons, sighed and added: ERASE. It never had existed.