if it was me i would work with pin 7 being the taco input to the clocks, my diagram is different to that of mattjk tho, what year did you select?
ben yes the wire does change colour at the bulk head, the wire colour at the clocks is white/blue or in a few cases it can be white. see what colour it is on pin 7.
TYhat diagram you have of the back of the clocks seems to be the led lighting for them.
Do you have a multimeter that has a continuity(spelling?) tester on it? if so put one end on the pin 7 and then try the solder joints arounf the tacho, see which one beeps thru.