If the staff is gone I would guess you need a new unit including the base. We have a member here who replaced his with a shark fin and if I remember correctly, posted instructions including pictures. Search the board.
The mast is available from GM as a separate item. Contact your dealer or Chris at Rippy Cadillac here on the forum. You can also upgrade to the 2009+ style antenna but it's quite a bit more work and costly than just the mast.
There are two styles. One is held on with two Torx screws and the other is a twist-on. They range between about $20 and $25 depending on vendor. Keep in mind that the mast is only for OnStar voice communications. The OnStar GPS and XM antennas are contained in the base assembly attached to the roof. Your biggest concern is moisture getting in and causing corrosion and damage, otherwise if you don't use OnStar your problem is primarily cosmetic.
On a 2008 STS I tried to match the tire pressure sensors using the directions in the owners manual by letting out air from the first tire but the horn doesn't chirp. What could be preventing the car from reading the sensor? Four wheels and tires were switched from a 2009 STS.