Before adjusting any cables/switches, I would advise you to check the rear brakes and service the calipers.
Flashing handbrake light is often a symptom of the handbrake mechanism in one of the calipers sticking.
There is a lever in each caliper that needs to freely move back under its own spring tension, and they often start sticking. If it gets too bad, handbrake efficiency deteriorates and new calipers are required.