Hi there! I also have a '77. The turn signal flasher is on the bulkhead above the glove compartment. Open the glove box and look above the bundle of wires running across, and there are two small boxes mounted to the bulkhead. One is the voltage regulator for the fuel gauge (has a purple and white wire and a green and black wire), and the other is the signal flasher. Hope that helps! Good luck.
-Dave