In a previous car as ridiculous as this sounds, it worked amazing. I used a center console cup holder to mount it. I mounted it to a 60* bracket to give it some tilt back, then I screwed the bracket into a hockey puck! The hockey puck then fit perfectly into my cup holder, and also let me swivel any direction I wanted.
My point is, first find the unit you think youll like the best, and base your decision primarily on the display and features, all brands are pretty much the same quality. I had an Audiovox unit which generally isnt top notch in audio, but mine worked great and never gave me a problem all of em have the same guts as prescribed by Sirius. THEN, after you get it, experiment with locations and fab something up to install it. While I dont recommend it for safety reasons, Ive even see people rig it up to their sun visors. You can get some itsy-bitsy units that fit in the palm of your hand, or some bulkier, albeit nicer displaying units.
Go to SiriusBackstage.com >> Forums >> Install Garage for ideas. Has pictures of peoples installs in various makes and models.