I don't use Sirius, but my 579 would randomly give me a signal message from the built in Sirius receiver. I never figured out where the antenna was, I even looked on top of the sleeper (that company didn't have the "anti-stow away" fairing cover on my year truck.) I thought it had to be in clear view of the sky to get a signal. I never removed the radio to see if it was hooked into the back like a regular receiver for fear of tripping some sort of anti-theft lock.
The 579's have a recall for this issue, just check into a pete dealer and let them know its giving you issues. They will move the antenna. Last I checked its located in the visor on the outside and requires it to be removed to get at it.
Again tho its covered, I had to have it done to mine took them about a hour.