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Hello. The tan wire should be the wire from the sender. You will have to ohm that to ground and move the sender or shake the car to make sure the resistance changed. If it does then you have the correct wire. Next test the other two wires. One should be ground and the other should be battery power.
As for the position on the guage threads , look at the back of the guage. You should have three studs. A left , a lower middle and a right. sender goes on the left. Ground goes on the lower middle. And power goes on the right. And again this is while you are looking at the back of the guage. Let me know if this helps. Lou
Also here is a link that I recommend everyone to use for old gm fuel gauges. Hope this information helps.