I had a similar problem when I purchased my 1979 Hunter 27 last spring. Every electrical device was dead except for one cabin light over the table. I'm no stranger to electricity so I promptly popped off the main panel to have a look and what I found was massive corrosion everywhere. The hot leads, the ground wires, the switches, everything, was covered in corrosion to the point where it did not carry electricity. I even popped off the lights to see what their deal was and found that the metal casing that grounds them was 100% rust. I had to run wires directly to the contacts on the bulbs to get them to work correctly. I also had to sand the rust off of every OEM switch (so I wouldn't have to replace them) and re-run MILES of wire all over the boat. I also had to replace the bulbs in most of my cabin lights, running lights (dead bulbs, faded lenses), the steaming light bulb, anchor light bulb, and the stern light bulb. In addition to those problems the PO had connected all the ground wires to a rusted bolt ON THE TRANSMISSION. IDIOT. I had to take that apart, and connect everything to a terminal block so it would work correctly. I also replaced my vapor tight switch because while the old one worked, it was rusted to crap and I doubted its vapor-tightness. It took me months to get everything right again.
Point is you either have a huge pain in the ass problem, or a simpler problem. However, from what you describe I bet you have a lot of work in front of you. Buy a multimeter and get to work! Good luck.
