Audi 80 2.0 ABS problems...

Hi there - first post, so please be kind!
I've got a 1992 80 2.0, ABK I think. Lovely car, but has an ABS issue - the warning light is now permanently on. Since I got the car three years ago the ABS light has popped on every now and then, stayed on for a couple of days, then gone off. For the last couple of months the ABS warning light has only stayed off once, and is now permanently on... Thing is the brakes are fine - no juddering, no binding or fade. Absolutely OK, with new discs & pads on the back. The ABS switch on the dash has never worked - rather, made a difference to the light on the dash. I had the sensors on the wheels checked a while ago - all OK.
So - question is how to put the light out? I'm not convinced it's the ABS unit (no other syptoms, as I've found searching this and other forums), seems more an electrical issue. I presume that an ABS warning light lit up is an MOT failure - shame as the car stops fine!
Any hints or tips on sorting this one very welcome - is this a common fault with an easy fix (other than putting a drill through the dash cover into the warning light...!)?
Thanks, Richard.