It's either low fluid, one of the sensors, or air in the system. At least that's what I assumed with my truck. I bled the system, checked the fluid, and ultimately ended up replacing a $10.00 sensor. Morale of the story (since it's not a Mustang) is that you should troubleshoot the previously mentioned things if you have time ... before you go pay $100. HOWEVER, if you don't know how to bleed brakes well, you should not try to do this step because if air gets into the ABS system you will have a helluva problem that will require the 'special tool' Ford has.
When your ABS light is on you still have brakes. My Mustang does not have ABS and I personally am glad ......