when i press the brake pedal the side headlight illuminates. Why?2003 audi a4 2.0 fsi sport

[magnumpi]

I've just noticed that when i press the brake pedal the drivers side front head light illuminates. Any ideas why. The car is an Audi A4 fsi sport 2.0.

[JohnyIre]

This could be a bad earth lead, you often get strange things happening when the earth is bad, or do you have the correct bulbs in?

[coupe2]

replace all rear bulbs this happened to me and when i replaced all the rear bulbs it sorted the problem

[Crasher]

You have a single filament bulb in a dual filament fitting so the power passes to the side light circuit from the brake lights, we see this all the time.