Like Dox1966 said, it's part of the bonnet latch
You can see it, covered in oil and dust, in the lower left corner of the lock assembly.
You will have to take the bumper off to get at it, though.
It's the only switch in that area, so it should be easy to trace the wiring back to the plug on the inside of the engine bay.
However, according to the manual the bonnet switch only resets the oil level warning, not the oil pressure.
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If lights up or flashes red (insufficient oil pressure)
Should the warning light flash or light up while driving you will also hear three times an acoustic warning as soon as the engine runs at over 1500 r.p.m. You must stop the car and turn the engine off: check the oil level and top up if necessary.
Lights up yellow (oil level* too low)
If the pilot light lights up yellow the level is too low. Stop engine and refill oil.
When opening the bonnet the oil level warning is reset. However, if you do not refill oil the warning will reappear after about 100 km.
Flashes yellow (defective oil level* indicator)
If the oil level indicator is defective an acoustic signal will sound and the warning lamp will flash several times.
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Have you had the oil pickup pipe looked at yet? IF this is blocked, then it is a known cause of low oil pressure and cleaning or replacing it will
help. However, you need to look at the strainer first, because if it isn't blocked then the
problem is elsewhere.
Any competent garage should be able to connect an oil pressure gauge to the car and tell you what the oil pressure actually is.
Oil pumps don't usually fail - they are running immersed in oil after all - but you may be unlucky. If the strainer and pickup pipe are clear, and the engine still registers low oil pressure, then the oil pump may need replacing. Check the drive chain and tensioner as well.
The oil pressure switch itself may have gone bad - a check of the oil pressure by an external gauge, as mentioned above, will highlight this.
The pressure relief valve on the oil filter housing may have failed (although it's difficult to see how). It is designed to open at 5 bar if the oil filter becomes completely blocked, keeping the engine lubricated albeit with unfiltered oil.