The additional GPS Antenna is recommended when your vehicle is fitted with an athermic windscreen.
Also known as a comfort screen, these front windscreens have an ultra thin film of silver (you can't see it!) running through your windscreen to reflect away UV rays from the sun. Good news for your car's interior, bad news for any GPS based system because it blocks the GPS signal to the antennae.
This additional internal antenna simply plugs into the side of the unit and you can run this to the back parcel shelf where the GPS signal can be picked up.