I have a MK IV golf and for reasons that I won't offend you with (to do with Empeg replacement ), I need to switch a relay from the dash illumination circuit. Because this is on a dimmer, the voltage on this circuit measures between 2v and 12v depending on the dimmer level. I know I could find a full 12v lighting circuit , but it would be a much neater solution if I could work off the illumination circuit at the stereo connection. Can anyone suggest the simplest way of achieving this.

On my Polo (´00) it's actually a pulsewidth modulated signal (IIRC ~85Hz) and wasn't suitable to drive a relay directly. If it's the same in your Golf you'd probably need a way to integrate the signal into something much slower moving, then maybe a comparator (preferably with some hysteres) to get distinct 0 and 12V levels. I'd look at some other wire if at all possible for what you want it to do.

EASY ! (if I understand you)
power the relay coil off the FEED into the dimmer, it'll be +12vdc. and only hot when the dash lights are on, no matter how dim you have them set.

by the way, many cars now use PWM to regulate dash dimming.

Thanks for the replies. I was trying to avoid having to use the feed into the dimmer, purely on the grounds that the "dimmed" wire is available at the stereo harness connector, but if the dimmer is a PWM signal that obviously complicates things. I'll just have to run a cable to the back of the dimmer switch.

Being stubborn and really wanting to only use the cables available at the ISO connector rather than tapping into other areas of the car, I have found a solution to this. If any VW owners are interested, it was very simple and just needed two components.

I tapped into the illumination feed wire (using a diode to prevent any current flowing back to the other illumination circuits) and connected a 100 microfarad capacitor between that and earth to smooth out the PWM signal. The output from this now measures between 9 and 11 volts and works the 12V relay at any dimmer setting. I did try using larger capacitors which produced a constant 11v output, but there was then a small delay between switching off the lighting circuit and the relay de-activating. The 100 microfarad gives virtually zero delay on switching off.