Remove your rear headlining. Disconnect the connector (You'll need to trace it to the front of the car and remove it) from the passenger kick trim by the big multi-plugs.
The once the wiring is free, simply under the locking nut of the mast, and it will then pull out of the car.
Feed the new wiring into the car from the outside, then check it fits into the existing hole OK before going any further. Then once your happy it fits, lift it slightly and run a bead of clear silicone sealant around the hole, fit it into place, and do the locking nut up nice and tight.
Run the new aerial cable to the front of the car and connect it into the extension in the passenger kick well.
Job done!
I did it to one of my saloons. I got a similar radio/phone/GPS aerial and drilled the roof out to fit it! Thats scary! lol. Vec B saloons don't have an external roof mounted aerial remember!