Ok the plastic pipe on the boost gauge from looking under my mates r33 comes from a T piece. The T piece is located sumwhere inbetween the return pipe that goes from ur BoV to the top of the block (not sure, its just a fitting that u put the hose on).

Broken down. Look for your BoV (stock or aftermarker). There will be a hose that goes from the BoV to your block. That hose will contain 1xT piece and a hose will T off into ur firewall. That is the hose the boost gauge needs to read the amount of boost it is running at one time.

Correct me if im wrong people, only going on my mates r33 and im fairly new to the whole turbo scene!