A 12V flasher will not work on 6 volts- - - -not enough current flow through the bimetal strip to heat it up and make it disconnect. The signal lights are probably coming on steady front and rear and not flashing, right? You'll need to find a 6V flasher to make the system work properly.

Note: most 6V flashers have a "P" terminal (pilot) which was intended to illuminate a single light on the dash that reminds the driver the flasher is operating. If you have an older pickup, it probably does not have arrows built into the dash cluster to show right and left turns.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!