Cleaning all of the grounds is a very good place to start. Once you're sure your grounds are good move on to looking for power. In the AF we used what they called the "halving" method. Start in between your power source and your system, roughly halfway; if you don't have power keep working toward the source; if you do have power work toward the system. Be very systematic in your approach, don't jump around and only change one thing at a time. If you change too many things at once you won't know what fixed it once you do fix the problem.
Electric is like water, it takes the easiest path. If you see an corrosion clean it. Ohming wires can help you find out if the is corrosion or a break in some hard to get to place as well.