There are actually two lights that come on under boost.
They work as follows;
When the boost rises above 2.5 inches of mercury the first setpoint in the round hobbs type switch is reached, it shorts the wht/blk stripe wire to ground to light the first light, when 10 inches is reached the second switch inside shorts the light blue/yellow stripe wire to ground lighting the second light.
Take a wire going to a good ground and a Key ON test take the connector off the hobbs switch and short each terminal to ground via the wire.
Each light should light up when grounded.
If they do the switch may be bad or not grounded well to the body of the car via the bolts, there are adjustable contacts for each switch but it requires some sort of calibration air pressure into the switch to set the levels properly.
Electronics contact cleaner may help if you take the unit off the car to clean it up or test it.