hia, just been trying to make some sence of a wiring diagram ( would post it up but dont know how ) and what i can gather is that the brown and red wire coming from the sender goes to the gauge. now looking at the connecter on the gauge cluster, pin 6 of 6 (brown) is the earth for the oil press gauge itself, pin 3 of 6 (red and brown) is the feed from the sender and pin 5 (black and yellow) is the power source from fuse 27.
i can only asume the second wire on the sender is the earth. i'm no sparky so i hope this helps mate