Just did the same thing not too long ago. Check out this page: http://jsalmi.com/car/boostgauge/
Basically, you can run the wires through a rubber grommet in the driver's side wheel well. You'll have to remove the wheel and liner to access it. The wires will end up coming out from behind the fuse box in the cabin. Use a grabber or straighten out a wire hanger (like I did) to make your life easier.