Re: 156 elecrical fault
If the car starts without problem then I wouldn't suspect the battery. If the lights are bright when the engine is running then I'd suggest that the alternator is fine.
It sounds more like a problem in your instrumentation display. Or does the warning lamp and the speedo share any common parts (I mean if you look at a wiring diagram is there anything common that if shorted to ground or else if it failed would cause this problem). Is anything else not working like the backlight on the instrumentation display?