Curious warning light for number plate light on an E30

[Jacko 2158]

If I turn on my lights with my foot on the brakes my number plate warning light comes on. The number plate light still works, and if I turn on the lights with no brakes on I get no warning.

Is this some sorta weird voltage drop/resistance thing confusing the warning light circuit? Do others E30's do this?

[hotwire 352]

Number plate light? There are two:-

Both working?

Otherwise - maybe a bad earth in rear lights