(not the truck in question, though visually similar!)

99 Ford Ranger 4 wheel abs. It's my Dad's truck, and the problem is, the abs stays on. Not sure when it started, but it came on sometime and killed the battery. We heard the extra sound when we connected jumper cables, and figured out that it was the HCU constantly running. We ended up pulling the 50 amp fuse under the hood to stop it. It was even running without the keys in the truck. Any ideas?