smoked boost gauge wiring confusion.....

Bear with this, it should make sense


im wiring up a smoked boost gauge so its black when switched off turn the car on and it lights up.

theres 3 wires :- WHITE 12v ignition (back of ciglighter)
Black Ground (stereo cage)
orange 12v dash lighting ( LED on cig lighter)


now its the orange dash lightiing wire thats confusing me as to why i need it ?
without it connected the gauge still lights up soon as you turn the key...

Now i connected it as said to the green wire on cig l.e.d and its on without lights switched but when i turn the lights ON the gauge DIMS a little bit

NOW is this correct operation do you reckon, at night switch lights on and gauge dims to avoid startling you in the dark ????

yep as normal, dims slightly at night and brigther during the day so you can see it properly............

alyn