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I would start with the antenna, once that is ruled out then move on to the headunit. If you had another antenna same make and model that you could play with then troubleshooting could go faster. Once the antenna is ruled out then you are left with the headunit unless there is another device between the antenna and headunit. Like Afugy said call XM...its been awhile since I had them but they may be able to test your headunit. Depending on model they maybe able to get you into some test functions or diaganostics to see signal strength...etc.
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