Once the airbox is removed, you should unfasten three more connectors.  The first is connected to the alternator (under the hose circled in red).  The second is inserted into the oil filter housing pointing towards the passenger side (yellow).  The third is also inserted into the oil filter housing, but points towards the front of the car (green).  (I took this picture after the fact so the pressure sender is where this last connector used to be.  The sender is that thing near the green circle with the blue wires running out of it.)  This last connector is the one connected to the factory pressure sender and feeds into the oil pressure idiot light on your instrument panel.  Remember this connector for later.  Now you should have acess to the factory pressure sender.  You'll need a 24mm socket or wrench to remove it.  Once it's removed, clean it and put it away.