[mydecember1985]

This is on the Diamante?
If it DOES light up then the bulb, wiring, and fuse are OK. Try plugging the wire back on to make sure it wasn't just loose. (You might get under there and find it just popped off! ) If that doesn't fix it then the switch is probably bad.

The Diamante engines only have the on/off dummy light. No big mushroom one for the gauge.
I traced the wire up to the 3 plugs closest to the throttle body. I unplugged it there and it had all this old electrical grease in it. I grounded that pin to the engine and the oil light came on.

So I knew the harness to interior was good. So then I got under the car and took the connector off the switch. Probed the connector and grounded to block. Wife said the oil light came on. So looks like the pressure switch has failed.
No manner of jiggling the connector at the switch would make it come on.

Found a replacement for either 4.79 or 5.99 at Advance Auto nearby.

 

[mydecember1985]

Hopefully solved... Let us know how it turns out!

Its is indeed solved. I wish all the fixes on these cars were $6 and 20 minutes or less. 😞

Thanks for the explanation of the grounding test.