To work properly, an aerial has to be grounded - i.e. the bottom of the mount needs to have an earth.
In a fixed aerial used at home, this will be the pole going down into the ground, but on a vehicle, the actual metal of the vehicle has to do the job.
Where your mount makes contact with the vehicles body, you will need to scrape a bit of the paint away, so that you make a good metal to metal connection.
On Land & Range Rovers, when the aluminium is exposed to air, a chemical reaction called oxidisation starts to happen. Over a relatively short period of time this can introduce a resistance to the earth connection. Without prevention, the connection is soon lost, and your aerial is trying to work without a ground plane, with a very noticeable deterioration in performance (and the risk of damage to the set).
One method you could use is to run a short earth strap (heavy gauge speaker cable will do) from the bottom of the mount to a point on the chassis or elsewhere on the vehicle.
There are a couple of products available that allow you to get a good ground plane without having to worry about scraping paint away, or using an earthing strap...