I actually just had this issue about two weeks ago when I got a new Sirius radio. The way I mounted the antenna was on the roof, towards the rear of the car in the middle. Then I ran the antenna wire down around the rear window using the existing molding to hold the wire. If you have a set of those tiny screwdrivers, you can use the flat head to gently push the wire underneath the rubber molding and it will stay there very nicely. Then I ran it into the truck and thru the back seat middle arm rest. You will have to remove the cloth backing on the far wall of the trunk but then you will see a small hole in the metal which you can use to fish the wire thru. Then go into the backseat and pull the wire through until it is relatively secure. I then fished the wire up towards the ceiling behind the rear seats and used the cloth moldings above the doors and then down the A pillar by the drivers side. I had just enough wire to make it. Sorry if that sounds convoluted but its really very easy once you are out in your car and looking at all the parts. It took me less than ten minutes and I had never done it before. Good luck!
Brian