I am away from my Bentley's right now but one of the wires (and only one) will supply a momentary 12v to the relay when the overdrive switch on the shifter is depressed. You can simply put a test light on each wire in turn and depress the overdrive switch and watch for the test light to light with the button pressed in (you need to provide a good ground for the light). If you can't get the light to come on with the button depressed you need to explore the reasons why. If the light does come on with the button depressed you have correct function of the switch.

Randy