You said the A/C compressor and relay work fine. Your problem is simply between point A and B. You should look closely at the plug. Get one of those inline splicers/piggyback connectors and clamp it in various spots of the wire between point A and B with a hot attached to it to narrow down your bad spot. I would start close the compressor to eliminate your most likely cause.. the plug
I do not fear what the future holds for me, I am not afraid to die.