Yep mine is in my trunk. I get reception problems in the garage and if I'm on a street covered with large trees. Havent been in a big city downtown, but I'm sure the tall buildings would inhibit the signal. I'd rather lose signal every once in a while than put the antennae on the exterior of my car.
My XM antenna is on my dashboard, centered and all the way to the front. No real reception problems. It's also one of the smallest antennas they make though. A bigger antenna might be too noticable/annoying.
I have got to believe that the Sirius antenna is th same as the XM. I have mine velcroed to the cloth lip inside the trunk lid close to the trunk clasp. Velcor works great. I would definitely put it on the driver's side of the car. There will be less interference there (i.e., closer to the centerline of the road).
I didn't do mine, but as long as you don't mind becoming somewhat of a contortionist, the install shouldn't be to hard. While you are it, putting in a rear partition might be a worthwhile project as well.