If you are 100% sure your bulbs are ok ,and that you are getting a positive feed .Just for a quick test ,to confirm your suspicions of a faulty earth , run a wire from batt neg ..If bulb lights then trace back to you find and fault
What i wil add about the use of a multimeter,is be careful ,as they can lead you astray .Though it may show 12Volts ,it isn't drawing any current ,it is just battery voltage ,putting a test lamp over the plug would draw current ...Was told that many many years ago as an apprentice when i thought everything was ok on a training aid ,to be told i was only reading batt voltage