Is there a GRY-BLU wire going to the flasher bulbs?

If you briefly ground that GRY-BLU wire with a jumper wire to the cabinet ground strap does it light the flasher bulbls?

Good, you have narrowed down the problem to something on the flasher board or the lamps wiring. Where you grounded the GRY-BLU wire should have lit the bulbs.

What does the GRY-BLU wire connect to on flasher board, can you post a picture of flasher board?

Another thing to try: Find the wire on the flasher board that's closest to and feeds to the lamps. Briefly ground that wire with a jumper wire to the cabinet ground strap does it light the flasher bulbs? If that lights up the bulbs, problem is on flasher board.