Does anyone know where the GPS antenna is located on 2001 LX470?

Also, I've noticed that at the base of my rear-view mirror, there is a seperate black round piece that is attached to the windshield. I am having hard time believing that this is part of the auto dimming rear-view mirror. Does anyone know what this is?

I am already getting the butterflies thinking about putting up the ugly Sirius antennae on the roof, let alone another extra GPS antenna mounted on the roof for the Pioneer Avic. I would love to just use the existing Antennae with the Pioneer unit. Any ideas?