I installed the same exact radio. The tuner includes the antenna. And it's super easy to install. Literally, plug it in.
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If you have an existing Subscription, they will want to charge you $15 to move the sub to the new Radio ID. Just tell them no, you would like to cancel, XM will send you a refund, and then you sign up again with free activation, and possibly a free trial till the end of the summer. At that point, they will agree to move the sub for free.
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You can look at my write up on this Fusion head unit. But it buffers XM, for track skip and also lets you do Tune Mix, which will blend multiple Xm stations for custom music mixes to help avoid the frumpys in the boat not liking your music.