Satellite radio provider Sirius XM Radio Inc. posted a sharp increase in its first-quarter net income Tuesday, fueled by a growing subscriber base that was due largely to strong auto sales. Sirius added 373,064 net subscribers during the quarter, ending the period with a new high of 20.6 million customers. That’s up 9 percent from 18.9 million a year earlier. Higher auto sales help Sirius because the company has deals with major auto makers to provide its radio programming in cars. For the three months ended March 31, the company reported net income of $78.1 million, or 1 cent per share, up 88 percent from $41.6 million, also 1 cent per share, in the same period a year earlier. The company had slightly more shares outstanding in the most recent quarter, which decreases per-share earnings. Revenue rose 9 percent to $723.8 million from $663.8 million.