Electrical Gremlin.....weird, man.

My 74 Dart is running like a top on Tuesday....everything works. Wednesday morning I drive her to work and the instrument lights aren't working. On the way home I get pulled over and the nice policeman tells me I failed to signal a left turn. After I tuck the citation away, I find out the rear blinkers horn, tail and brake lights are inop. And the plate light is out. And the side marker lights aren't coming on.

What the heck is goin on here ?

What IS working you ask ?
Headlights, backup lights, front turn signals and wipers. And the radio.
What circuit, relay, fuse, ground or switch do these 7 things share? Or did I have 7 things fail at the same time?
My Haynes is kinda weak in this department.