I have Sirius in my Insight. I almost got in to an argument with the Best Buy sales guy who was trying to tell me the magnetic antenna would stick to the alluminum roof. I contacted Sirius to see what they said, which was if using tape to hold it down I'd need a mylar strip to properly ground it to the body. I proceeded anyways without. I got some 3M side molding adhesive tape (pricey, but it holds forever) and put some pieces on the bottom side of the antenna then carefully cut off the excess with a razor blade so it looks good. I then lined it up with the radio antenna and stuck it directly behind the radio antenna on the glass. You would basically safrafice any use of your rear wiper nozzel, but I never ever use it anyways, and I wasn't about to mount anything that wasn't absolutely necessary on my precious paint.
Anyways, I ran the wire under the hatch window and then up under the weather seals. I have the brain mounted in the styrafoam block where the factory CD changer would go, which I hollowed out to fit the brain unit in. Then under the rear carpet is a channel that the CD changer wiring would run through. Then the wire goes under the side trim by the seat and under the scuff guard plastic piece (where there's another wire channel thats unused), up under the dash and over to the deck.
Hope this helps, the 3M molding adhesive tape can be found at auto paint and body shops. And you can also try mounting the antenna inside the car as well first.