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Source:SiriusXM.com.

Satellite-radio service Sirius XM (NASDAQ:SIRI) has been successful in persuading automakers to build its technology into their cars and trucks, and it can now boast having its receivers in some 70% of new vehicles that roll off the production line.

That bodes well for the company's continuing efforts to introduce new listeners to what it has to offer. Free trials are extended to new-car buyers, and Sirius can work directly with the manufacturers to improve its rate of sign-ups.

The less obvious but much larger opportunity, though, lies in the used cars already on the road. Increasing numbers of those with installed satellite radios will turn over each year to drivers who haven't heard Sirius XM before.

That's why CFO David Frear didn't have to hesitate when asked in June about the company's biggest opportunities over the next five years, telling analysts that "it's definitely the second-owner market."

In fact, company executives believe that with these satellite-equipped used cars rolling over to new owners, the number of cars with installed Sirius XM technology will more than double over the next several years.

That number now stands at around 73 million. Sirius executives say they believe it will reach some 160 million within the next eight years.

Used is a bigger market than new
This is an especially important aspect of Sirius XM's story right now, a time when new-car sales are on the decline while user-car sales are on their way up. Kelly Blue Book was expecting August new-car sales to come in down about 4% from last year. TRUECar, meanwhile, was expecting a corresponding 3.8% increase in monthly used-car sales over last August.

But percentages tell only part of the story there. Used-car sales outpace new sales by well more than 2-to-1, with more than 3.5 million anticipated used-vehicle sales to about 1.5 million new cars and trucks in September alone.

As the next several years unfold, far more potential Sirius customers are going to be introduced to the service through used cars rather than new.

Sounds great for Sirius. But then why aren't executives anticipating a fast ramp-up in subscriber growth?

New challenges in used cars
The big reason is that the used-car market also presents significantly different challenges for Sirius. Where it can work directly with car companies on the new-car side, implementing programs that can be executed through the car makers' systems of dealerships, the used-car market is infinitely more fragmented.

Some buyers are purchasing those vehicles from franchised dealerships. Others are buying from regional chains, or small independent lots. Still others are driving away on used wheels they discovered on Craigslist or in ads in the local newspaper.

In fact, research from CNW Marketing Research last year indicated that nearly two-thirds of the 42 million used vehicles that changed hands in the U.S. in 2013 were sold either at lots that weren't part of franchised dealerships, or by individuals.

That makes reaching out to those buyers and getting them to activate their free trials a much greater challenge for Sirius XM than it's been with new-vehicle owners.

Efforts under way
So how does it get to those used buyers? As of earlier this year, Sirius had been working with about 17,000 U.S. auto dealers in its efforts to reach used-car buyers with trial offers. But that's well below half the dealerships in the United States.

Sirius has been exploring a number of other avenues. The company had signed up 7,000 dealers to offer trials to customers who bring in their cars for servicing. It's been talking with the companies that finance loans for used cars. It's been speaking with auto-insurance providers to see if they'd be willing to let the satellite-radio company work through them to reach out to owners.

It's also been exploring options with those companies whose websites and apps serve as intermediaries in the sales of used vehicles.

Maybe not a boon, but a driver of growth
Sirius grew revenue last quarter by 8% over the prior year. It grew subscribers at about the same rate. The satellite-radio service will soon be nearing 30 million subscribers in the U.S., and a growing number of new-car buyers will have already been introduced to the service and made a decision as to whether it's worth their money.

Investors shouldn't expect growth to accelerate from here, at least not significantly so. But with a growing number of used cars turning over to owners who may never have tried out the satellite service, there remains a long runway to maintain steady subscriber growth.

It really depends on how well the company can effectively reach out to used car buyers and turn them into listeners.

John-Erik Koslosky has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.