You can check for earth lead fault with Multi meter, set to 12\24 volt DC
Neg lead on battery -, meter + to chassis or engine do not connect before helper is cranking there should be 0 to 1 volt MAX showing while cranking or running, you would be surprised at variation from model to model.
If connections cleaned & no change suspect under speck wire, have seen 3 volt drop on cars from new, cheap penny pinching cost engineering :bang:
If item is earth return there should be NO or minimum voltage from its case\ earth point to battery Neg terminal.
We always checked battery- to earth as part of a service also in the distant days distributor case to earth another high resistance spot.