I fixed my stock boost gauge by making a new vacuum hose connection at the boost sensor. My gauge would work intermittently, sometimes the needle would rest dead all the way to the left inside the gauge. The first pic is underhood, right front corner.
The boost sensor is marked by the black X. The vacuum line to the boost sensor is T'd into a vacuum line that also goes to the AIV solenoid, circled in green. The AIV is circled in red. Unscrew the bolts circled in yellow to free the boost sensor from the mounting bracket. .
The next pic shows the way I remounted the boost sensor AFTER I after I did an AIV delete. That is why the vacuum line is capped. The AIV system is gone already.
The important point is to replace the vacuum line at the boost sensor, circled in white, and as far back as leaks and cracks may exist in the old rubber line.
The AIV does not have to be deleted to do this fix.
The boost gauge will usually start working again by simply trimming off the old cracked flared-out end of the vacuum tubing shown at the white circle and then reconnecting it. Be careful when replacing or removing the vacuum tubing from the boost sensor. Do not bend or twist the long metal nipple underneath. If your vacuum hose is original and stuck on, carefully trim off the old tubing with a sharp knife and replace it with new vacuum tubing, preferably silicone. This will fix most dicey stock boost gauges.