Yes, AC harness. It's separate to the main harness so not a major drama to replace. Annoyingly, there's a plug just on the other side of the grommet but looks like they wanted to keep firewall sealed. There were a few variations and changes during 77 and early 78 with changing of switch locations and air sensors, but can be made to work. Just need to confirm with other harness layout/plugs if it was early or late harness. Don't go off shop listings without confirming elsewhere. I noticed even when looking this up that they have another ad of similar but wrong pic.
Do you have a 77 wiring diagram/Service Manual?
It would have been similar to this. In 77 they introduced different engines and layout so kept adjusting things to suit instead of one single harness.
New air conditioning wiring harness for your 77 - 78 Firebird or Trans Am. This is the engine compartment section. You may also need the compressor jumper wire harness shown below to complete the circuit.
www.firebirdcentral.com
This would be on the dash side connected to it, runs to heater controls and main harness. 78+ is very similar but a wire or two in different locations so best to check plugs and arrangement to know which version you have. I think it was a late-77 change rather than day of new model.
This is a correct reproductions of the original air conditioning wiring harnesses which were used on factory optional air conditioning. Each harness will duplicate the original in every detail including correct color wiring. Note: For use with factory original air conditioning systems only...
www.classicindustries.com