Birds nest? Do you mean someone added onto the wiring? Like maybe for trailer lights?
Like Shoprat said, corrosion is a big thing... so don't just check the socket for voltage and then declare the circuit dead, check voltage on the wires behind the socket too... look for any mickey moused wiring, poorly done connections.
And remember grounds are just as important as the power wire! Bad/loose grounds can make your lights do weird stuff... and I do mean weird!
First things first, get yourself a meter if you don't have one. You're troubleshooting blind without one. I was helping a kid on another forum with his stereo wiring in his truck and i swear he couldn't get it in his head that he needed a meter! Start at one end and work your way to the next, on down the line; no skipping around!
If you have some pictures that helps us help you too. Since the rear lights are not the same across manufacturers its hard to tell you to look for such and so colored wire.
Hope we can help ya!