Ok...in the interest of saving a future searcher some time: The sirius retrofit kit does NOT go under the flap that raises in the trunk. You have to take out the whole thing.
It goes under the part near the back seat. I pulled the whole thing out of the car and there was the spot for the sirius kit as well as the nicely wrapped wires for the satellite receiver.
I was asking if it goes on the black plastic area under the flap but I apparently didn't make that clear.