Paul just a thought based on some work done on on my S2.
Where the bunch of cables go through the door (adjacent to the hinges) into the body behind the front wheel is often tight and this can cause the cables to be damaged. When the speaker cables were upgraded on the S2 they opened up the slots and put in fresh rubber protection around the edge of the hole. Some of the cable did show signs of wear so they wrapped and protected them with more tape.
It might be worth inspecting this for random shorts?
