Trace the wires and look for the short. Its an intermitant problem. (sometimes it starts and sometimes it dosent).
More than likely a bare spot on a wire intermitanly touckes metal like carnut said. A couple of inches of electrical tape and wala.
The section of harnes suppling the starter and oil pressure switch is most vunrable near the hot exhaust manifold. Its dark and hard to see the harness from topside. Go under and shine light up and past the starter. not much room to work, but the starter drops very easy making for enough room to manuver to disconect the starter and pressure switch. Pull the harness up topside to carfully inspect the wires, removing the harness sheath.
This is what I had to do just to inspect find and repair the short in my fuel system. I started at the tank, removing the pump and bench testing it (it was a new one). Then I meter tested each run, conector to conector (pulling interior panels, thresholds, seatbelts, seats) and still didnt find the short until I inspected the last and only remaining run to the oil pressure switch.