right so you know the realy o/p is good & that the compressor is earthed.
if you look under the upper cove rof the engine the wiring runs along under it.
due to the heat it possible that the wire has become brittle & has broken.
in other word, work back form the compressor, checking the single wired connector just at the compressor itself & then back along.
it may not be physically broken to look at but broekn inside the insulation
if you know the color of the wire, at the compressor, then try find it D4B, the 6 pin connector..... is that the one at the bulk head?.... if you find it do a continuity check from D4B to the single wire connector