Oil light not working

There is zero chance that adapter is not grounded. But it's easy to check the light I already told you how.

1....Ground the sender wire. Turn the key to run. Does the light work?

2....Put your meter on low ohms. Stab one probe hard into the block/ pan/ somewhere to get ground. Or clip it to the battery NEG

Stab the other probe into the adapter. See if it shows continuity