LS4 Performance - Need a new Antenna Mast

Anyone know where to get one or the part number.

And I obviously know the dealer sells them, would like to get it a few bucks cheaper if possible.

06 GP GXP with Onstar and XM.

Antenna looks like this:

http://www.sandyblogs.com/techlink/may_06_techlink_fig9.jpg

And apparently this is what I am missing:

http://www.sandyblogs.com/techlink/may_06_techlink_fig10.jpg

Somebody was kind enough to borrow the upper portion.

Thanks,

John

http://www.gmpartsgiant.com/parts/gm-antenna-25912948.html

Or is it?

1 **335503 ANTENNA M/TEL DIGITAL,(76.7 MM),(SHARK FIN AND SCREW ON STYLE) W (UE1) 06-08 1
$28.08

$17.20
*
http://www.gmpartsgiant.com/online/Page_Product/PartDetail.aspx?modelYear=2006&model=Grand%20Prix&make=Pontiac&catalogCode=2W1&modelCode=2WC69&seriesCode=2WC&bodyStyleCode=69&engineCode=L32&transCode=M15&majorIndexID=09&isBigPicture=False&minorIndexID=2W0903701

It's not screw on. I have one that screws on on my truck.

The part for the top is 10335503.

OnStar Antenna

Part Number: 25775305 (Base) 10335503 (Antenna)

Alternate Number: 25842452

There are so many types?!! Why couldn't GM just use one?