I replaced mine recently; unscrewing the old aerial from the wing and screwing in the new one is fairly self explanatory.
I then cut the old cable and taped up the new plug to the end of the old cable.
If you take the radio out (2 x 2.5mm allen bolts in the small holes on either side of the unit for the standard radio/cassette) you can then unplug the aerial from the radio, set the radio aside and gently use the old cable to pull the new one through the grommet in the bulk head. It's obviously a tight squeeze with the old cable and new plug and cable, but if you have a second pair of hands you should be able to ease it through.
You're then left with the new cable and plug to go in the back of radio. Screw that back in and you're all done.
It might be a bit of a squeeze working through the radio hole, but it's much easier than taking the dash apart.