Now that I know where the radio connection is, I ordered the hardware to patch the Sirius radio into the car. Principly I need the direct FM adampter from Sirius, but also two adapters to adapt Ford's non-standard antenna connector to it. Both pieces of hardware should arrive next week.
For now, I disconnected the cable from the car's normal antenna and I taped the transmitter wire from the Siriuc car kit across the antenna jack on the radio box in the trunk. It works really well; we haven't heard any static at all on it. However, we can't listen to terrestrial radio with the current setup.
At the bottom right corner of the radio box is the antenna input connector. Just to the left of it is a long, multi-pin connector with no cord connected to it. That's where an optional trunk CD changer plugs in when it's installed.