... take off the ground wires one at a time, clean them with a wire brush, and reinstall them. Them go around and do the positive wires and do the same.
A very effective tool for this, with a very poor "return on investment," is a battery-powered Dremel Moto-Tool with tiny steel wire brush wheels.
The power tool is made like crap (or at least the one I destroyed ten years ago was), but it will work long enough to clean every single connection on the car (don't forget the fuse block terminals!), and the wire brushes ain't cheap, and don't last long before they're bald, but, man, that thing works great for badly corroded little automotive connectors.
Be SURE to wear your safety glasses, though, as those wire wheels spray out tiny wires like #9 birdshot, right at your face.