does the voltage drop happen all the way back to the battery or only just after the key switch?
stays 24+volts on the line from the fuse to the switch? (bad key switch if only after the key switch)?
trace that voltage drop backwards to find where the voltage loss is.
I predict you find a bad fuseholder or fuse, although the over the crosshead/in the mast control wire cable is a very likely suspect too.
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