Another alternative...
I used a generic antenna cable extension combined with the cable from the OEM antenna, and ran a line up in to the roof skin.
The cable runs under the dash, and up the right hand side a-pillar under the trim panel. The cable's shield is grounded to the frame somewhere above the passenger seat; the remainder of the cable run has had the shielding removed (but the core insulator left intact) and looped around the rear of the sunroof hole between the space frame and the roof skin.
Works nicely; should be do-able in sunroof'd cars as well (mine isn't) but may require loosening of the roof skin studs in order to get the cable between the skin's sunroof frame and the space frame.
Got photos at home, somewhere. Will try to remember to dig 'em up & post.