Hi Michael,
The oil pressure switch is just under/behind the alternator/Power steering pump.
I suggest you just un-plug the connector from the switch and connect it directly to a good ground. This should keep your oil pressure LED ON all the time. If it doesn't it suggest there is something wrong in the wiring from that Oil Pressure connector to the instrument cluster. Try cleaning all the contacts in that circuit - starting with the Subaru engine connector.
Good luck,
Tom
Burlington ON
