Sirius XM Radio Inc (NASDAQ:SIRI) made its earnings report for the first three months of 2013 public earlier today. The digital radio provider narrowly missed analysts’ estimates for its revenue but still managed to bring in more cash than any past quarter. There may be one weakness in the shares that investors are not taking into account, and it’s buried in a new Citigroup Inc (NYSE:C) research note.

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The Citigroup Inc (NYSE:C) research note contained all the key metrics from this morning’s earnings report, but, being a short form report, contained little analysis besides. Under the risk factors section, the analysts list slowing auto sales as a risk factor for the stock. The bank is right, and it may be an aspect of Sirius XM Radio Inc (NASDAQ:SIRI) that investors in the firm are not giving full weight to.

After the release of this morning’s majorly in line earnings report, stock in Sirius XM Radio Inc (NASDAQ:SIRI) rose by more than 3.5 percent to $3.18 per share. Citigroup Inc (NYSE:C) puts a twelve month price target of $3.60 on the equity. Citigroup values Sirius based on a 22.4 multiplication of the company’s estimated free cash flow.

One of the reasons behind the recent surge in Sirius XM Radio Inc (NASDAQ:SIRI) shares is the increase in auto sales in the first quarter of the new year. The firm’s satellite radio has become the gold standard in entertainment in cars across the United States but, though there are some secular trends in its favor, the firm may run into problems if that industry slows.

To the advantage of Sirius XM Radio Inc (NASDAQ:SIRI) is the replacement of cars in the used car market by more modern models. That means that more and more mid-priced cars now come equipped with a satellite ready radio. That’s good for Sirius, as it gives the company a chance to add subscribers.

The problem for investors is that Sirius XM Radio Inc (NASDAQ:SIRI) stock is incredibly vulnerable to changes in the auto market. If that market changes, the company is likely to spin off its current growth path. Sirius XM Radio Inc (NASDAQ:SIRI) shares have risen more than 40 percent in the last twelve months.

Sirius XM Radio Inc (NASDAQ:SIRI) investors should be aware that the satellite radio provider is exposed to both the weaknesses of the auto industry and that of the entertainment industry. The company, in one sense, makes auto parts.