well i dunno if anyone has done this with their gauge, but it sure is fun! fun to drive too for an 4eat at least.
http://img187.exs.cx/img187/9753/interior6mt.jpg
it is actually adjustable still! not nearly as much though.
my vent was already broken, the on/off door in there broke and was jammed, so i gutted the vent and poked a hole in the duct for the wires/hose.
just had to show my engine bay too, you can see where the gauge line comes in. Is this where other people tapped into? looked best to me.
http://img204.exs.cx/img204/2712/enginebay3cx.jpg
look close you can see my boost controller (homemade) and Target Co. coffee mug
edit: just got the movie uploaded, sorry for the crap server, it works.
http://www.putfile.com/media.php?n=12psi
Enjoy!