First your batteries are not a "Dual Six Volt" setup or rather they shoudl not be, They shoudl be a 12 volt 2 piece set up, Like this -{6v}+-{6v}+ = 12 volt
Think of them that way
now, tool, You need only one for this job, a 12 volt automotive test lamp, this is a device that looks like an old fashion ice pick with a wire coming out of it, a light bulb is inside the clear or colored but transparent plastic handle.
Step one: hook clip to battery Positive post. touch tip to battery negative (Ignore the jumper that connects the two batteries, go for the wires that run off to other than the other battery) Bright light good. dark bad.
Now touch tip to unpainted metal frame part.. Again Bright light good, Dark - problem with ground lead, Touch ends of ground wire, clean and/or re-do bad connections.
All good (So Far) move clip to any good metal frame/chassis point, test by touching Positive post of battery, Now move out along the POSITIVE wire. If you find a device test both terminals, if it has 4 test the large ones. (forget the small ones for now) When ever you move the clip re-test the last known good point.
You can extend the ground (clip) wire if you wish as well
Problem is between BRIGHT and Dark.
