Just incase he means to remove the whole unit
Pull off the bit of trim at the back, pull down the roof lining and detach the aerial, then get some mole grips on the nut and undo it. The pull from the outside and it will come off
sorted.
Instead of mole grips you could always use a 22mm spanner or socket as it fits quite nice!!! i changed mine at the weekend and it now means i have working radio!
Samurai sword job ?