Typical air conditioner low voltage wiring...
From DC Fuse Block,
Power goes to FURNACE first,
then branches off to go to the
ceiling locations of each air cond.
Power wires in ceiling then will
connect to control boxes / relay
boxes for air conditioner. Tstat
will then get its power from
Given that your were running
the furnaces, I would start my
search at the furnaces themselves.
What I would look for is the power
wires rubbing against shroud of
furnace and grounding out.
There really isn't a lot to go wrong
within the ceiling assemblies for air
conditioners unless using heat strip
on ducted air conditioner.