2011 B8 Avant DAB upgrade

Hi peeps!
Ive just picked up my first Audi..a 2011 A4 Avant..it has the MMI 3G basic system installed but no DAB radio..it has sat nav and bluetooth etc...Now, i want to put DAB in it, but dont know the best way to do it...I have read that the DAB ariel is located on the rear drivers side window panel, and i do have an ariel there...is it a standard item they fit and you have to upgrade to dab?
Can i buy the parts to do it and is it expensive? Sorry for the questions...lol