Put the hazards on and see if it still flashes weak, if so check the circuit in the rear light, normally they get wet with condensation and earth out through the wrong strips, look for anything that may be resisiting current flow to the bulb.
The best way to check it is an earth is to cut a piece of wire long enough to touch the earth point on the light board for your bulb and then earth this on a good earth point like a bolt lock bolt or bumper bolt.
