It's not just Optimus Prime that watches over a fleet of battle warriors on wheels. Sirius XM Radio (Nasdaq: SIRI) has its own wheeled army, and it's getting bigger: The satellite-radio giant revealed today that it has surpassed 50 million factory vehicle installations of its Sirius or XM receivers.

The milestone isn't necessarily a surprise, though. Roughly 70% of all new cars in this country hit showrooms with factory-installed receivers.

Most car manufacturers have their allegiances. Ford (NYSE: F) offers Sirius receivers. General Motors (NYSE: GM) offers XM receivers, and it was an early investor in the company. However, it all ultimately bubbles up to a single parent company, ever since the combination of Sirius and XM four years ago.

There are now 22.9 million subscribers. A few of those are stand-alone online subscribers. A few more tune in through portable receivers, either on the go or at home. Many longtime subscribers who couldn't wait went ahead and had aftermarket receivers installed in their vehicles. However, the vast majority of listeners happen to be subscribers because their cars happened to come with XM or Sirius receivers.

However, this still means that more than half of the satellite receivers that were installed as cars rolled off the assembly lines are currently dormant. Sure, some of those cars have been junked or totaled along the way, but there's still a golden opportunity for Sirius XM to grow its account base without having to worry about wedging in costly receivers.

Sirius XM knows this all too well. It has spent the past couple of years striking deals with dealers selling used cars to offer three-month trials to hook new listeners buying used rides. It also offers aggressive pricing to win back former customers.

Sirius XM is never going to win everybody over. It's a challenge even with new cars, given the company's 46% conversion rate. In other words, 46% of new car buyers with Sirius or XM become paying customers at the end of their trial subscriptions.

The beauty of the Sirius XM model given its high fixed costs and low variable expenses is that winning over the half of the cars with dormant receivers will be a more lucrative opportunity than its current user base offers. Succeeding won't be easy, but there's a lot of money to be made if it does.

I also just put out a premium report on Sirius XM Radio, detailing the challenges and opportunities that await investors that are both long and short the dynamic media giant. A year of updates is also included with the report. Check it out now.

The Motley Fool owns shares of Ford. Motley Fool newsletter services have recommended buying shares of General Motors and Ford and creating a synthetic long position in Ford. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.

Longtime Fool contributor Rick Munarriz calls them as he sees them. He owns shares of Ford and is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Motley Fool has a disclosure policy.