FWIW the system you guys are describing with the 'switch or relay' on top of the batteries sounds alot like my dad's old Kenworth (except his was switching between 12 and 24 volts to start). The relay unit in question was called a 'parallel switch' by the local truck parts store. If you want to try that route that name might help (dunno, might not be the same nomenclature) if they have one set up for 6/12 as I don't know if a 12/24 would pull in on a 6/12 system.
