I searched far and wide in the manuals and diagnostics procedure. This seems to be an impossible behavior. This would indicate wrong/broken wiring somewhere between the light's relay and the rev counter...
Make sure your lights really light properly (meaning light up only when expected of them) and the correct bulbs were used. Try to turn the key the other way, putting it in park, see if the sidelights are operational, and if the error light is still on.
An alfa dealer would probably try changing your rev counter too... though im sure it can be fixed. It's almost as if the error LED was fitted in the wrong position (inverting + and -)!
Basically, you need to try and find another problem - because I never heard of this issue and neither do the repair manuals!