Boost Gauge
First make sure you have the right gauge. At idle the engine will be sucking air into the cylinders, so the gauge must read vacuum as well as boost. A range of +/- 1 bar is best.
The gauge will come with a length of tube and a T-piece. There are many places you can T into, but only one right one! Find the brake servo. There is a black hose (1" diameter) running from the right half of this towards the left hand side of the throttle body, into the inlet manifold. Follow the hose from the brake servo until you reach a one way valve with a T-piece in the side, then another T-piece with a white PTFE tube coming out of it, towards the ECU. DO NOT T into the ECU line as this may affect the reading to the ECU. At the first T-piece there is a cotton braided vacuum hose coming from the T-piece. This runs above the inlet manifold, past the throttle body. T into this line with the provide T-piece.
Next locate the accelerator cable grommet in the rear bulkhead. Below this is a second grommet which should have a large enough hole to allow the boost gauge hose though it also. Feed the hose though this into the area containing the heater unit, ECU etc., covered with a plastic cover. Remove this cover. In the bulkhead at the back of this section, between the battery and the heater is a hole with a grommet in it. Remove the grommet and cut a hole in it to fit the boost gauge hose. Fit it over the hose, then run the hose through the hole and refit the grommet. The hose will come out underneath the dash to the left of the steering wheel.
Finally choose the location of the gauge. I have mine on a small metal bracket fitted so the gubbins on the back of the gauge sit in the hole to the right of the cigarette lighter. The hose comes though the dash and straight into gauge. The wires for the backlight of the gauge go through the dash behind the cigarette lighter, and splice in the lighter light.