It's a right faff of a job. After you pull the rear trim off and drop the head liner down enough to get a spanner onto the aerial, you'll then notice the factory aerial harness disappears off to areas that are inaccessible unless you drop the whole head liner down.
Another thing to consider is a shark fin is a big advertisement to thieves that you have a nav unit, which they like to steal and sell on ebay. You don't need a shark fin for an RNS unit anyway. The GPS receiver sits under the top HVAC vent. You're better off getting a phone mount and use that for Nav instead. RNS radios are sluggish and the maps are woefully incomplete.