My autogauge sender came with a white and a black wire...so, baring that in mind...
As far as i remember, the illumination wires were physical wires which came out the back of the gauge, and then there were 3 bolts:
1) Positive - needed a 12v positive (switched)
2) Negative - any ground inside the car
3) S - wire to the white wire on the oil sender
The black on the sender was earthed, and it then worked nicely.
From wot i can make out from your wiring above, you have earthed the - and + bolts on ur gauge.