Half the battle is knowing your tester is making contact and a true reading - just a dumb light but you need to know it's either powered or grounded to know that the test will give you helpful info.
A common safety pin well sharpened helps with probing the back side of plug sockets. Sharpen the "awl" part of a test light too such that you can poke a wire (and usually your finger

) too OR run the pin right thru a wire.
Note: When making any violations to wire insulation I strongly suggest a drop of like clear nail polish OR some effective seal for even that tiny hole. Liquid electrical tape - anything that works. Just the least moisture over some time makes for more hair loss there which is why so many techs go bald!
When you know a wire is supposed to be ground with a safety pin effectively thru that wire can then be jumper clipped to good known local (body) ground and if all things work you know that's the issue.
Again if already said: To test that something is ground you need to power the clip on test light with known 12v from the car. If all else use jumper cables to get to the back direct from battery.
Keep at it and keep your HAIR!
T
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Tom Greenleaf - MetroWest, Boston
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