There's nothing sadder than a used car with an inactive satellite-radio receiver, at least to the ears at Sirius XM Radio (NASDAQ:SIRI). Now, courtesy of a new deal with Volvo, maybe the satellite-radio provider won't have to sing the blues quite as much.

The "boxy but good" automaker will offer three-month trials of Sirius programming to buyers of certified used Volvo cars with factory-installed receivers.

The free deal isn't as sweet as the six-month trial that Volvo is offering buyers of new 2010 models, but it's better than nothing.

It doesn't matter if the vehicle in question is a shiny new car or a serial lease vehicle that has exchanged hands a few times over the years. If it's been outfitted with a Sirius or XM receiver, Sirius XM isn't benefiting if the receiver is dashboard deadweight.

Volvo isn't the first company to promote satellite radio to used-car buyers in exchange for a piece of the action. Sirius XM has deals in place with Audi, BMW, and others.

These are smart deals, and Sirius XM may as well milk the market while it can over the next couple of years. There will come a time when auto subscribers are likely to max out. Once car buyers are trading in their satellite-radio-equipped cars for newer models with factory-installed models, this is likely to become a zero-sum game. Those who subscribe to Sirius or XM on their trade-ins will probably continue, while those who bring in cars with dormant receivers probably won't stick around after the free trials expire on subsequent purchases.

There are advancements that will tug at both ends at that point. Enhanced receivers with new features will make satellite radio more indispensible, yet in-car online connectivity will also open up the options to rival Web-based ear candy. 

We're still years removed from that tug-of-war. Don't let the past two quarters of declining subscriber counts fool you into thinking we peaked last year. The recession, automaker slowdown, and higher monthly fees have combined to cost Sirius XM a net loss of 590,421 subscribers through the first six months of the year. Sirius XM should bounce back from that, although the ascent will probably be slower than the descent.

Angling for subscribers in the used-car market should help. Resale specialists America's Car-Mart (NASDAQ:CRMT) and CarMax (NYSE:KMX) have held up better than the new-car specialists at AutoNation (NYSE:AN) and Group 1 Automotive (NYSE:GPI).

Until everyone is educated on the merits of premium radio, these deals are purely incremental.

Other items on the radio dial:

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Longtime Fool contributor Rick Munarriz is a subscriber to both Sirius and XM. He owns no shares in any of the companies in this story and is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.