Nav GPS Connector
The enclosed antenna in any of the major player GPS units is more than satisfactory for any aftermarket unit I have seen or used.
I have used Magellan and Garmin GPS in a boat and in 4 cars of ours and they all work well due to the number of satellites present. Just keep the GPS unit on the dash where it has a "look" out of the windshield and you have no need to connect into the old Nav unit antenna. I have never had a "lost signal" interrupt in the boat or car and never have used an external antenna. It is not uncommon to have as many as 8 satellites displayed on your unit, sometimes more--and it only takes 3 for navigation data. Trust me, go no further with your search until you try the internal antenna.
Jim