Extremely logical... I told OP to do that back in #5
The only way the chassis can actually be "live" WRT to supply -ve is a high resistance path to Chassis from the Supply.
That could/will be why his 12 Volt instrumentation is playing up, it isn't getting 12 volts WRT to chassis anymore.
On a 24 Volt system the chassis will appear live from the RH battery -ve (it will be 12 volts WRT earth obviously).
But if the OP is connecting a test lamp to LH side -ve and putting the probe on the engine or chassis and the lamp is illuminating then his "earth" isn't an earth IE the earth cable is high resistance.
It could be the engine to chassis earth.
His mention of the globe being brighter at different points screams earthing issue but as we have all mentioned, the only way to go is start at a logical point and go one step at a time.
Best check will be to put the test lamp on the chassis/engine end of the earth cable and go back to battery -ve.
Light on = battery cable faulty
Light off = battery cable OK
If light is off then go to Engine and Chassis and wherever the lamp glows is the broken earth strapping
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