I have XM in my '96 T-Bird (different body model than yours.) Mine, however is Delphi SkiFi2 that rebroadcasts the signal to an empty channel on the radio. (I installed an upgraded Kenwood radio years ago - before "satellite ready".)
However, I did buy the XM equipment for a satellite-ready radio head in case I upgrade to a bluetooth phone, and could then get the phone call through an upgraded bluetooth radio receiver.
What you need (for XM) is a XM tuner, and then a radio interface for your particular radio head (I assume OEM Ford). The antenna is a magnetic mount - typically, people with convertible run it back onto the trunk lid - it only needs to be about 4 to 6 inches away from the windshield on the body. I've even had mine inside the car and it worked pretty well.
for your 2005 T-bird, apparently this $130 kit will work:
XM Direct for Ford XMDFOR100 / XMDFDK
XM Satellite Radio at MyRadioStore - XM Radio Receivers - XM Direct for Ford XMDFOR100 / XMDFDK
for anyone else who is thinking of upgrading their T-Bird radio (I may or may not do it again) - be sure to check out this Audiovox XM Direct2 Universal Mini-Tuner - you can re-program it if you change cars or radios - but it only appears to work with Sony or Alpine head units -
XM Satellite Radio at MyRadioStore - XM Radio Receivers - Audiovox XM Direct2 Universal Mini-Tuner