Mine is inside my rear window brake light casing. The casing is plastic and doesn't interfere with the reception in my experience, but then I have a Garmin GPS35, which is really good. It works inside my metal trunk too..
But anyway, the brake light install is completely stealth.
Player: Celeron II 633MHz, 256MB RAM, 20GB IBM 9mm 2.5" Laptop HD (180G/2ms), onboard ethernet/sound/video/tvout, 10"11"x3" case, MPBS1 70W DC-DC PS w/auto-shutdown controller, in-dash lighted switches, 7" NTSC TFT widescreen in-dash LCD, touchscreen, rear-window brake light installed Garmin GPS35 GPS, credit card sized IR remote w/IRMan, mini-wireless keyboard/mouse (sits under seat), PowerMate black knob, MP3s and GPS Navigation (Winamp, CoPilot, SA8.0).
Car: 1993 Nissan Maxima, Black Emerald
Take a look at the link in his sig, it sits right in the middle front of his dash. And I've got the new wee garmin module that I could only get working on my front dash, so I simply painted it black and taped it in the corner.
Since when is insanity a bad thing? www.mp3vw.com