problem with ABS

My kid has a 99 sunfire and we're having issues with the ABS system. I understand the pulse and how the brakes should work. First off the drums, discs and pads are all good. On dry pavement the car stops great. But when on an icy surface, the ABS engages but it's real jerky. The back wheels lock up, on/off.
The front doesn't lock up but it sure isn't as smooth as my other vehicles.
For an experiment, I was going to pull the fuse to disengage the ABS system, but couldn't even find that in the fuse box.
Any idea what would cause this??? Wheel sensors, the master under the hood???
How about where the heck is the fuse?

Any help would be appreciated...

OK, I found the fuse under the hood. The ABS system is now disconnected and the brakes work great on all 4 corners. So at least I know the problem is with the ABS.
Actually, the car stops a lot safer now without the ABS.

So anyone out there have an idea what to check??????