We have a winner! The correct antenna adapter from an OEM Nav System stereo antenna plug to an Alpine requiring a Motorola male connector is Metra part number 40-VW12. It's a small fitting that costs all of $4.99 at Crutchfield (their part number is 12040VW12).

If your aftermarket stereo requires a Motorola female connector, then I have no idea which adapter you'll need. If anyone wants to buy my other two antenna connectors listed in the first post just let me know because I'll never use them.