When you retro fit, you'll need a separate new GPS antenna. If you get a unit off Ebay the seller should supply one of these for you. I fitted it under the centre dash grill (the one that looks like a cool centre speaker but isn't) but this has a metal content and totally destroyed reception. Now it's in the top corner of the windscreen under the headlining. You'd never know it was there.
Easy peasy and nothing to do with your original aerial.