Oil pressure gauges.

I have twin 120 Lehmans on my cruiser. I have 2 sets of instruments, one within the wheelhouse and the other on the bridge.
On the bridge the oil pressure gauge on the STB engine has stopped working. Looking on the engine I note that there are 3 sensors linked to the oil galleries. On the stb side in the centre there is a large switch with 2 contacts. On the port side there is a tapping just behind the alternator which has a Tee piece with a switch at each side. On one tee there is a switch similar to that on the port side and on the other tee there s a much smaller switch.
Of course the wires disappear into the wiring harness and become untraceable.
i therefore need to know how to trace which sensos might be the one that goes to the oil pressure gauges and on all switches what voltages, with the power on or off will I see? Any other suggestions as to how I locate the fault either in the gauge, the wiring or the sensors are most welcome.
Thanks for reading.
If the gauge in the wheelhouse is working, the issue is on the bridge or the wiring connecting the two....unless the two sets of gauges are wired independently.

It would be good to learn the differences between 'switches' and 'senders'....gauges are usually connected to senders.
The sender in the middle of the engine with two terminals is oil pressure. Behind the alternator with two terminals is alternator energizing switch and single terminal is oil alarm switch.
Charlie W