My dad has a 2004 LT, where is the factory GPS antenna? I want to stick a Sirius antenna right beside it
If it's a BMW Navigator II GPS (a Garmin 2610), the antenna is built into the unit. If it's the older, original BMW Navigator (Garmin StreetPilot III) (unlikely with a 2004), mounted over the dash, the antenna is separate, and is under the dash "dome," velcroed (yes, velcroed) to the top/back of the instrument cluster housing.
You may want to use the search function for Sirius antenna posts -- there have been lots of original mounting ideas over the past couple of years.