I have put a 71 ss dash in my 72 chevelle. I have the 71 dash wiring harness that came with the dash. My question is my 72 has factory air and the 71 I got my dash and harness from did not have air. What will I have to do to get the harness I have to work with my a/c dash. I was hoping maybe GM made just one harness with the wiring for the a/c and just did not use the wire for the a/c on non a/c cars. If not does anybody know how to add the extra wires for the a/c?

If you have a gauge dash harness it is set up to accommodate either heat only or A/C heat. If you have a indicating "Idiot" light dash harness, it is not set up for either. However it is an easy modification. No extra wires are needed.

Now- The 71 dash harness will not work, without major mods, in your 72. It is not uncommon for a seller to state "70 17 72 Chevelle dash harness" This is not the case. 70 and 71 are, for the most part, the same, 72 is significantly different.

The 71 ss dash that I have is a gauge dash with the original wiring harness that came with the dash. (I took them out of the donor car) It was my understanding that all I had to do was get a 71 engine bay harness to match up with the fuse bulk head. Are there other modifications that I don't know about

The big difference between 70/71 and 72 is how the main power comes into the dash harness. In 70/71 power is run across the front of the car, to the horn relay next to the radiator and into the forward lamp harness bulkhead connector.
In 72 power comes from the starter, to the horn relay mounted next to the brake booster, and into the engine bulkhead connector.
As Elree stated it won't work without modding it. First thing you will find is that there is no power inside the car. The car will be dead.

That aint cool. So would I be better off modifying the 72 engine harness or modifying the 71 engine harness. You guys have been a great help thanks a ton.