It's been a while since I had a dash out, but I'm pretty sure you HAVE TO take the instrument cluster out to access a couple fasteners behind it. It's not too hard. Everything is routed to one large "connector" that is really just a rectangular opening with the printed circuit board draped over both sides. Terrible 1979 technology. A lot of g body cars have instrument cluster problems due to this poor design. On one of my early Malibus, I did away with this printed circuit nonsense and ran wires to all the gauges and used a huge connector for everything that I got from Radio Shack. Was a long process, but I never had problems with anything after that. Before that, I had intermittent problems with the lights, fuel gauge, and one other gauge I don't remember at the moment, lol.
**That bulkhead connector at the firewall is just for the engine wiring (engine compartment) ONLY. No interior wiring is on that connector.