I got a complete wiring harness from Speedway Motors for my rat truck a while back. I do not know who makes them.
I was going to get the 6 circuit harness from them, but when I was ready to buy, they had the 21 circuit on clearance for 99 bucks. duhhhhhh
I am very impressed with the quality. Of course, it;s GM color coded. Came with all the connectors already installed and even several switches, including a GM style push pull headlight switch.
I have not installed it yet, but it's comin real soon.
Oh and there is no firewall connector. .