Ford Motor Co. will begin offering Sirius Satellite Radio Inc.'s service as a factory-installed option in four vehicle lines this summer and will increase that number to as many as 21 vehicle lines in the next two years, the two companies said.

Beginning this summer, Sirius will come factory installed on the 2006 Ford F-150, Ford Explorer, Lincoln Mark LT and Mercury Mountaineer.

Ford, Lincoln and Mercury are targeting an additional 17 models for the factory-installed option for the 2006 and 2007 model years, which would make it available on 80% of all Ford, Lincoln and Mercury models.