I mentioned this in another thread for XM, and I don't know if the Sirius antenna is small enough, but...
I installed my XM antenna under the plastic piece at the front edge of the hood. You will have to buy replacement clips because you have to break the existing clips to remove it, but they were only about $12.
Just remove the (~8) leading edge nuts, (2) back nuts and (~7, the center one is orange, the rest are blue) clips from under the hood. Take off the plastic piece.
Here's the fun part. Take a rubber mallet and bang a dent into a spot on top of the hood that was under the plastic, big enough for the antenna to fit after the plastic piece is re-installed. Put the antenna in the dent. Run the wire inside one of the hood gussets down to the hinge. Tape the wire to the hinge and run it down to the firewall so that it can't get pinched when closing the hood.
Run the wire thru the steering column rubber grommet, there is an extra nipple that can be cut to create a little pass thru hole. Then your wire will be under the dash in the drivers foot-well.
Check that the Sirius Radio works before putting the plastic piece back on the hood, in case there is a problem, you won't need more clips.
Then once the plastic piece is reattached you can't tell from the outside that you have Sirius, so less chance of someone breaking in your vehicle to get your radio. And no worries when washing. And the location is perfect for getting signal, even under a carport, as long as you back in.