Finally installed my boost gauge over the weekend!!
I decided to place it in the left vent, using Steve Schwing's special gauge adapter.

Here are some pictures along with instructions on how I went about installing it:

Here's the gauge in the box:




In my case, HPA had already added a T-adapter (not sure where to add it for 1.8T engines though):




Attach on end of the provided tube to the T-adapter:




Locate the grommet in the firewall




Here it is, simply remove it by pulling on its side:




Make a small hole in the grommet (little smaller than the tube's diameter):




Pass the other end of the tube through the grommet, keeping in mind which way faces "out":




After removing the lower dash panel from under the steering wheel (check my sig for details under item 10), replace the grommet with the tube - here's the inside part of the firewall:




Here's a view of the vent mechanism from the underside of the dash:




Here's the left vent, you'll need to remove the top (front) half of it:




It's fairly easy to push it out by grabbing the right ends:




Keep pushing the vent until it becomes free from the tubing:




Here's the open vent hole, you'll need to pass the tube and electric wires through the top side:




Pass the tube over the pedals and through the fuse holder:




Pass it over the white vent tubing:




Connect the ground (black) wire to the star-shaped hub under the steering wheel:




Pass the two wires and tube through the vent:




Now to the hardest part: pry the vent apart by pushing on the ribbed edges:



I found that pushing on two edges at a time makes it a lot easier.


Here are the three parts you end up with after the vent is disassembled:




Disconnect these two pieces from each other, no tools required: