On my 86 300sdl it was the right front cable that goes to the sensor on the wheel hub. They take a lot of abuse over the years. The point where it went bad was where the cable sits in the bracket by the brake line. It squeezed it for 21 years and my cable was broken there. My symptoms were ABS light on when car starts and then goes off. As soon as I drove above 10mph the light would go on.
I found two nice cables on E-Bay and replaced both and it is fixed.

John