If you look on page 8-144, of that manual you referenced, "Dart inst. panel wiring", here is how the flasher gets power. It's a bit roundabout, and misleading. The power lead for the flasher is D1-18BK. Follow that to the bulkhead connector terminal W. That terminal is used as a junction for the flasher power. Out it goes on B1-18W. Follow that over and up and you'll see that it ends at the RED wire terminal of the radio connector. They are merely robbing power from that (radio) circuit. Now follow the red, X12-18R, and it goes over and off the page to the right. Continue on the other page, 8-145, coming in off the left of that page. Finally, you'll see that connects to the bottom of fuse "C" in the fuse box. Look to the tops of fuses ABC and you'll see a buss. That is the "ACC" buss, fed from the ignition switch, and is live only in "run" or "acc" positions of the key. To the right, fuses DE are fed off the "hot buss" hot all the time. and the last fuse to the right, "F" is a trick circuit for the dash light dimmer circuit.