When I asked Subaru to install the sirius adapter on my radio I asked them the same question. I'm guessing you are going with a plug and play kit? Is this going to be hardwired or are you using the AC power. Results do vary depending upon how you will run your receiver.

My Subaru's radio came with a satellite option, it just wasn't installed on the one I was looking at. I asked them about the installation of the module and they specified it would only be the antenna visible for roughly 3 inches up the passenger side of my windshield. However, I opted to go with a satellite ready headunit as they wanted over $500 for the module and installation.