My Sapphire GPS mouse looks like a little black dome, quite inconspicuous as it is
However, I have mine under the top part of my dash, next to where the clock is and the heater vents. You cannot see it at all, but it still picks up 6-8 satellites, and fixes into position almost before the software has finished loading!
Putting it in my boot, it picked up maybe 2-3 satellites, and took forever to get a fix.
They view to the sky with no metal in the way. Fabric and plastic don't affect it (if plastic did, then it being under my dash would get it pick up nothing!), only metal.
Ideally, somewhere in/near the dash would be best, as the windscreens normally give you a good view of lots of the sky, which is what you want
Garry