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.