Im a huge fan of the Kuda Base, Its expensive, but looks like ford put it there itself
http://www.kudausa.com/ford-focus-si...ack-a1666.html
a lot of others have gotten the pro-clip mount which clips onto the vents, but that is also a bit on the expensive side.
http://www.focusfanatics.com/forum/showthread.php?t=262345