Actually, they don't HAVE to. GM owns a significant stake in XM, and I beleive they have one or two people on their board.
And the current status quo is doing quite well. With GM, Honda and a few other car manufacturers, a satellite radio option exists for a number of models as an option package. If someone wants it, they can add the option in, and not only do they get a radio, but the first year of service is built in to the purchase price of the car (exactly the same model as OnStar). Once the subscriber is "hooked," they're likely (as has proven true in previous SEC filings) to pay for the service once the first year is up.
Not everyone opts for this though. Personally, I think GM needs to reduce the size and improve the appearance of their satellite radio antenna. It currently looks like a big ugly black wart on the roof of the car, regardless of the car's paint color, and usually the placement isn't even centered, so anyone who is interested in preserving the lines and look of the vehicle will have issue with it. On the other hand, the aftermarket satellite radio systems have a tiny magnet mount antenna no larger than a postage stamp, and you can get creative as to where you place it. I opted for aftermarket, placed the antenna on the trunk lid where it's not too distracting, and got the added bonus of having a removable unit that can be taken and used indoors as well.
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