There is a "tuned" adapter cord (from Sirius dash/windshield mounted unit -various Directed Electronics "Sirius" branded models) from one of the parts sites. The one that I noticed that Autotoys.com has some adapters (
SHOPPING FOR CAR AUDIO, SECURITY AND AUTO ACCESSORIES. AUTOTOYS - If I see additional sites, I'll post them.
In the mean time, may I suggest that you contact Toyota customer care and ask if they are going to sell a new radio (head unit) that is Sirius or XM ready for both the Yaris and Corolla.
There is a satellite ready radio available for Scion, but I believe it is only the xB.