That's where your fault is. Check everything that connects directly to the
the connection point between the two rear batteries.
I believe one of the 18 volt taps are there isn't it? I do not have a
schematic in front of me at the moment.
I know I have a ground fault now (it's insane: I can measure voltage to
frame even with the main switch off, however the only place it is at zero
volts is between the rear two batteries. Everywhere else it's a function
of
6 volts up or down the ladder) so this was probably a double fault.