I had similar on a BMW I owned a few yrs back. Tried and tested everything I could find/think of but cldn't find it. Auto electrician spent some time on it and found a snapped off drill bit was melted across the wires in the A pillar/footwell ans was causing a short. Previous owner clearly tried to drill the A pillar (speakers I guess) and snapped the bit off. Pure chance that it fell on the wires and being hot melted through, but worth a look as you seem to of tried the rest?
cheeRS