Hi, I do not have the XM Radio in my Highlander, but I do have it in my Infiniti I30 so maybe I can help you out with your Highlander?
I personally do not know how big and bulky your receiver is, but if you can, you may want to try and mount it in one of the floor bins in the rear cargo area. Or, what about mounting it behind one of the side walls in the rear cargo area? In order to do this, you would need to just pop off the side moulding in the rear cargo area and see how much room is back there. If there is enough room then this might be the best place for it because it will not get in your way and it will be hidden from view. The other place you may want to try is putting it right underneath one of the front seats.
I don't know if this helped or not, but feel free to email me with any other questions you have and I am sure we can make this work.