One of my most useful tools for detecting electrical earth issue is a long length of electrical wire with a small crocodile clip at each end.
Connect one end direct to battery earth terminal & the other to the item you're testing. If the item now works check & clean its own earthing method.
Often dirty/corroded earth point which just needs a good clean.
If it still doesn't work then check with a test bulb that there is sufficient current flow in the live feed to the item concerned - it can sometimes be the case that corrosion in the wire reduces the amount of strands & whilst a digital test meter will show battery voltage present there isn't enough to power the item & a test bulb won't light. It's the load of the test bulb that shows it up.
Last time I had that was the in-tank fuel pump on my RRC stopped working & the end of live wire at the loom connector was badly corroded.