It's not just the surprisingly robust 2012 net subscriber additions revving up Sirius XM Radio (SIRI -4.75%) to a four-year high this week.

The satellite radio provider also announced earlier this week that Toyota (TM -3.65%) will be ramping up the availability of Sirius XM receivers across its car lines.

Satellite radio will now be a standard feature across most of the Japanese automaker's multimedia systems, including all of the vehicles equipped with Toyota's Entune system.

Sirius XM investors may be familiar with Entune. Toyota introduced it two years ago during 2011's CES, raising the bar on car multimedia platforms. Shareholders had a right to be concerned. Entune seemed to give smartphone owners every reason to cut their premium satellite radio subscriptions loose. Two of the six icons on Entune's home screen were for Pandora (P) and Clear Channel's (NASDAQOTH: CCMO) iHeartRadio. Offering voice recognition and in-vehicle controls to work seamlessly with Bluetooth-enabled wireless phones, it gave drivers free access to the ad-supported music-discovery features of Pandora and iHeartRadio's hundreds of radio stations. Even those lacking smartphones had more ear-candy options through the HD Radio stations available through Entune.

We all know how that played out. Despite Ford's (F -1.66%) MyFord Touch and Toyota's Entune cracking open the audio entertainment possibilities for smartphone-packing drivers, Sirius XM didn't miss a beat. The media giant tacked on 1.7 million net additions in 2011 and a whopping 2 million net new users in 2012.

In short, making it easier to stream a growing array of content in the car hasn't crushed Sirius XM's popularity. The only time that Sirius XM did suffer a dip in subscribers was during the first half of 2009 when auto sales were sputtering during the recessionary crisis.

We're thankfully in a much better place now. Auto sales are booming. Industry tracker Autodata reported last month that sales of cars and light trucks were at a five-year high. Things are going so well that Ford moved to double its quarterly dividend this morning.

More cars -- and particularly more new cars with factory-installed Sirius or XM receivers -- are the lifeblood of Sirius XM. It's flowing well these days, and it's not a surprise to see Sirius XM growing more in tune with Entune.