Since I didn't use the AM/FM part of my TT's built-in stereo, and the antenna was directly above the stereo, I removed the AM antenna, fished the Sirius antenna's wire through the hole to the stereo compartment and then out a hole I drilled in the cabinet, to the car kit base. I also lucked out that the stereo had an AUX input in the back, so the Sirius car kit output goes through the same hole and plugs into the AUX input.
As BurbMan did, I also glued a fender washer(to the sub-roofing in my case). I used Dicor all around to seal it. I haven't had a leak in 4 years. Of course, I check it every year and add Dicor if it even looks as if it might leak.