Those bars ground through the chassis/frame and the rotators have a single power lead that can be wired together.
The flasher cans I have use black wires to ground the bulbs to the can and power (red) wires that power the bulbs and run to an insulated connector to the side of the can:
If you are using an alternating flasher, connect each lead of the two cans to the load terminals of your flasher (L & P on a 537). Connect 12 V negative to the bar frame and you're in business. My diagram uses a single switch point to power rotators and flashers together. Using two switches, you can power rotators and flashers separately.