Re: 1966 chevelle dash harness
You can also use one from a '67, but there will be slight differences from year to year. There will also be differences depending on the options of the car the harness came with. Auto trans, A/C, factory gauges etc. all determine which connectors the harness came with.
If you have a car with gauges & you find a harness from an idiot light dash, you won't have all the wires & connectors needed to make it all work. I have modified a couple of harnesses over the years when adding gauges to idiot light cars, but it's tricky to find the proper connectors if that's important to you.
If you don't have a harness at all, the new harnesses from M&H are a great product. I've used their forward lamp harness on a '67 & it was really nice. AAW also has a great reputation, but I've never used their products. In the long run you'd be much better off with a brand new harness than a 50 year old used piece that you don't know the history of. Just my 2 cents.