I'm looking for a diagram for the dimmer. Trying to get a boost gauge with a light on it sorted so I can dim it according to brightness outside. Currently had it hooked into the light switch but it's just too bright and ends up stopping me from seeing out the side mirror. I need to know the exact wire for the positive I'm tapping into. Have it on the gray/blue wire, but that doesn't seem to dim it. Not got a multimeter either which don't help...

The Grey with Blue tracer is the lighting output wire from the dimmer control.

If your boost gauge has LED backlighting they sometimes don’t work well with variable voltage they tend to stay at full brightness until the voltage drops below the threshold and then stop working

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Gotcha, I'm pretty sure it's LED that my boost gauge has. Would there be any way whatsoever that I could link this up to have the desired effect?

What colour is the backlighting in the gauge? and how dim do you want it 50% dimmer? more or less ?

10 hours ago, nige8021 said:

What colour is the backlighting in the gauge? and how dim do you want it 50% dimmer? more or less ?

Just white tbh mate, comes on pretty quickly so I'd assume it's LED. As for the brightness, tbh just enough so I can see out the mirror lol, 50% would probably do.

Try the grey wire going to the light switch instead.

Put a 1.2Kohm 1/4w resistor in the line to the gauge that should reduce the current by 50% making it dimmer if that isn’t dim enough then increase the resistance but don’t go more than 2.2Kohm or it won’t work

@nige8021 , and @rj_BNM , the wire colours at the light switch are confusing relative to the colour code that generally provides illumination feeds.

The grey/blue ( (in red box)) is the power from the light switch going to the dimmer switch, and rear plate lamps.

It's the grey wire at pin 17 (in green box) that is the controlled, dimmable feed from the dimmer switch to the illumination of the light switch, and therefore the one to t-off for your new gauge.

Edited 1 October, 2025Oct 1 by Pete1

@Pete1 that is very strange it is only on the 9N1/3 that the output from the dimmer unit is grey on other models pre and after it’s grey/blue on other models in the VAG range also use grey/blue as the dimmable output I’ve got part of a loom for my 08 Audi A6 and the backlighting wire is grey/blue

Unfortunately the OP hasn’t specified what model they have

Could be an error in the wiring diagrams, I guess.

I no longer have a Polo to look at. ☹️