FYI, just wanted to update.

Used the VIN window sticker generator and saw where this originally did have Sirius installed. It does have the antenna on the roof over the passenger side. However, pressing the "SR" button on the radio said "See Dealer" or something like that. The ESN was NOT displayed where the manual said (lower left corner of the System Info screen where the Navi DVD version, etc. are shown.

I used the instructions RI-Matt linked to (along with trial and error) and found that the module IS installed and the antenna is plugged in. I unplugged both harnesses and the antenna, and checked all the related fuses - all were good. I guess somehow that resest the radio because when I tried the "SR" button again, it acquired signal lock and started playing 90's on 9! The system info screen shows the ESN and all is good!

Now, just need to find out who is still paying for the sub! (2007 Expy we bought used about a year ago...) Maybe we shall just enjoy it until it stops!