Hi,
If your rear light cluster is the type that uses a common earth on a removable panel from inside the boot (i.e. all the light bulbs come out at once and are mounted on the panel).
You may have corrosion which is causing a short across from the brake circuit to the indicator circuit.
If it is it's easy to fix with a bit of wet and dry or fine sandpaper to clean the corroded areas and a squirt of wd40 to reprotect.
Cheers
westcountry, October 2013
