As others have mentioned its probably an earth fault.
Take the bulbs out of the rear light cluster and give the contacts a good clean on the bulbs and the holders. that usually solves the problem
What is happening is the earth to one of the bulbs (probably the brake light)is poor so the current is taking an easier path to earth and this just happens to be through the indicator bulb causing it to light.
cheers one of the metal connectors was bent slightly therefore not touching,bent it back with a screwdriver and bobs your uncle