I found a pretty trick firewall connector to pass wiring from the engine side to the cabin. Since I'm doing an LS3 swap with all modern gauges I didn't need the original engine wiring harness at all. But removing it leaves an ugly 2" hole in the firewall. Then I found this neat bulkhead connector from Maven Performance. I selected the single bulkhead connector with 4ea 12AWG pins, 19ea 16AWG pins, and 6ea 20AWG pins. The connector comes with the required male and female connector pins. This connector fits as if it were made for that ugly 2" hole. All I had to do was a little clean up sanding of the hole and it dropped right in. Then just drilled the 4 holes and secured it in place with the provided hardware. Since the mounting plate is what attaches to the firewall, the cabin side of the connector can be accessed by removing the large hex nut on the engine side of the firewall. The cabin connector can then be pulled out for any changes to the wiring and the mounting plate stays in place. The connector is keyed so it will go back in the same orientation and will not rotate.
This connector uses the Deutsch pins/sockets. The Deutsch pin style, both solid and stamped, is becoming more commonplace in newer cars. The solid pin style is used with this, and many other, connectors. This particular firewall bulkhead connector is completely sealed and weather tight.
This is what you get, the mounting plate, the cabin side connector, and the 1/4 turn engine side connector. It comes with all the pins necessary for all 29 connections. Other pin configurations are available.
I also ordered the boots both straight and 90 degree. They are not heat shrink but fit the connector quite well. The plastic pins are for the unused pins.
This next part is important. The pins are mil-spec like the Amphenol or Cannon connectors on military equipment. Accordingly, they require a specific crimping tool. I spent over 35 years working for the US Air Force (as a civilian, not active duty) and have a pretty good background on the engineering/technical side of aviation electronics. If you want to use these types of connectors whether they be mil-spec or not, I STRONGLY recommend buying, borrowing, or renting the correct Deutsch crimping tool. It's expensive I know, around $260 for the solid pin tool used here, but you will not be sorry in the end. I have seen people try to get by with the Chinese clones but they invariably wind up buying the real thing. So save your money, get the good tool the first time. Many of the Packard 56 and 59 style pins can be successfully crimped with the "all in one" crimpers with the exchangeable jaws, although it can be a bit tedious. Once you use this crimping tool for solid Deutsch pins, you will never go back! You can crimp a pin with 100% success faster than you can read this last sentence. Plus, the Deutsch connector pins are becoming more and more common on newer cars. They have a variety of plastic shells instead of this 1/4 turn bulkhead connector, but the pins can be crimped with this tool as long as you use the solid pins. Do a little research, Maven Performance is a good place to start. The crimper below is a Deutsch model HDT-48-00 and will handle 12AWG - 20 AWG.