As far as I can remember...
The plastic hinge cover is unclipped (with a bit of difficulty).
The aerial base is the same shape as the cover you've unclipped, and clips on just the same as the piece you took off.
On the last one I swapped over, the lead was just run through the gap near the hinge when the door is open, and there's enough "give" in the door seal rubber to accept the thin lead which is routed along the "parcel shelf" to the back of the radio.
There may well be a more clever way of getting the cable into the car via the hole where the hinge bolts are, but I can't say I've done that in living memory:I The books I've got give no clues (although Porter usefully suggests leaving the old cable in, and re-taping the ends together if you replace an aerial, which I suppose means there is a cunning and difficult route for them, when done "properly")
HTH
Regards
John H
