a/c and heater wiring

[68r/t]

Hope someone can help me out here, I bought a new wiring harness for the the a/c and heater and have plugged it in but I have one black wire with white tracerand a grey  insulator left over in the new harness.
The wiring diagram that I have says that it goes to the instrument cluster but I don't know where.
Any idea where it may go?
Another question I have is now that got the harness in, the fan goes when you put the a/c on then speeds up when I push the fan speed switch which i presume is correct, now when I push the heat and the demist switches the fan does not go, is that possibly a problem with the vacuum switch or maybe the black wire needs a home?
Any help will be much appreciated.