2000 626 with ABS light on

For a while the light came on only once an a while, but now it is on all the time. I feel the first place to look is at the "sensors" at each wheel, but I'm not sure where they are, or what they even look like. Can anyone give me some help? Where can I see a photo of the location of the sensors? I'm guessing they might just need some cleaning. Thanks.