How do you fix the brake alarm on a 1993 GMC Topkick?

The brake alarm kept going off for no reason. Then I would pull over and put the truck in park to shut off alarm. Then I would drive again and then again the alarm would go off! Fuild level is at max and brake shoe and pads are fine. After 7 months of this the parking alarm stopped! Then when I park on an incline of any degree the truck just rolls, so I have to put the wheels into the curb to prevent it from moving. It goes fine forward or reverse but the parking brake, which is a yellow toggle switch on the dash, does nothing now. And when I put the truck into the park position, that doesn't do anything either now? I've read that maybe it's an actuator messup or a blown fuse, since the brake is under electric power not air brake. So now I need to figure out how to make it work again. Help!