There is rubber grommet right in front of the driver (in a right-hand drive Z at least!). I pushed the pipe through the grommet and routed it under the dash with cable ties. Then up the side of the dash where the door closes. My gauge is mounted on the window side of the instrument cover thing so it was pretty easy to hide 99% of the cabling.
dont drill holes in your dash.. i bought a 3pillar gauge for my shiro.. ran the wires under the dash, then between the door and dash, into the back of the pillar..if you put your wires in colored loom the same color as your dash you will probably never notice them on a dash mount... personally i would shoot myself in the foot before i drilled a hole in my dash.. and velcro/sticky never looks good after you take it off... good luck~
the three guage pillar i have takes the place of the factory pillar.. the 3gauge is attached to metal, not the old pillar, although there are two self tapping screws that go into the metal... it was easy to install and it didnt hurt anything as far as the two screws..