ground out relay leg
Just prior to you suggesting this, I ground out the leg of the relay (actually penetrated the lead wire about 6" from the relay) and as expected, the fans came on. Ground out the br/bl lead at the pressure switch, no fans. When I measure the resistance between the two points (relay and pressure switch removed from the circuit) I get 148 ohms.
So...if I'm getting some resistance, this must be the two ends of the wire even though the colours are different. That I must confess still bothers me. As the schematic says there is no other junction between these two components, I believe I'm going to patch in a new wire and hope for the best.
By the way Arthur, I am using the green 30amp relay.
Regards and thanks to all.