The wire could be the oil pressure sensor wire as it is next to the filter. In the photo (this isn't quite like the standard set up as it has a sandwich plate) the pressure switch is the silver and black item at about 4 o'clock in relation to the filter housing, with a wire going to it. If it is disconnected the oil pressure light will not operate correctly. For info there is a section on posting photos in the "Welcome Forum" at the top

[Linked Image]