I know on buses, they typically use a Flasher unit feeding a Bosch Relay to get the flashing lights. Not sure if there's a 6-volt Bosch relay or not.
There also is such a thing as an Alternating Flasher unit, which would be simpler, as it does it all in one unit. Here again, don't know if a 6-volt alternating flasher exists . . .
-Michael