If you don't have teleaid, I doubt that the antenna is installed. I know it is not in my car. If you do have teleaid, the GPS antenna is located in the top right (passenger) side of the rear glass. It is behind the plastic molding. It can't be seen from the outside because of the black band at the top of the glass.
Not exactly an answer to your question, but - I got a magnetic external antenna from a guy on eBay called GPSgeek. Cost about $25 and works like a champ. If you consider this option, be aware of the drain the antenna has and make sure the connector fits your GPS. It would probably be a good idea to ensure that the MB antenna is compatible with your Garmin before you start into your headliner.
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