fast blinker and loss of power

When you washed your Stang, did you also wash out the engine bay? If so, you've probably got water in the wiring harness.

A fast-flashing bulb could also mean high resistance on the lines, even though the bulbs are working. You might have gotten some water in a connector. Though I admit I'm not sure what else would cause the bulbs to blink fast.

As to the voltage meter, sounds like you have a bad ground wire. On the driver's side, there should be a wire going from the driver's motor mount to the frame near the sway bar. Disconnect it at both ends, make sure there isn't any corrosion. I'd also recommend replacing it anyway - the factory ones are known for going bad.