1974 Plymouth Duster no power through bulkhead.
A lot of changed items and some very non stock items.
Post some photos of the engine bay wiring and we can figure out what is there, and then walk you through finding the problem.
The photos most helpful will be
Battery to starter relay
Inner fender and firewall showing ECU, ballast resistors, and multi-connector.
That's also where the seatbelt interlock overide switch will be - if its still present.
Additionally lets see the wiring to the fan and water pump - and if you can where that wire connects.
Finally, a photo of the instrument panel because the factory panel didn't have voltmeter.
I will try to get some photos of it today, it is at my father-in-law's house currently while trying to get it going.
The aftermarket items, I do know, have been wired outside of the harness with in line fuses and breakers and power supplied from the starter relay post, aside from currently the one aftermarket gauge that I explain toward the end of this reply. The tach is currently unwired where I replaced the harnesses. It was the only thing spliced into the harness when I got it and just the power wire on an ignition power source.
I have tried the reset on the seatbelt interlock, where I had seen that in another thread, but I have not bypassed it.
The wiring currently is new "plug and Play" from classic industries and matched everything to a t on wire color and connectors. The only issue with the wiring harness is that it didn't come with the wiper motor harness. I know this is a stupid question, but is it possible that that harness is leaving some circuit incomplete, where it plugs into the bulk head connector and currently is not there?
On the subject of the "voltmeter" I meant the alternator gauge in the cluster. It has alternator, fuel, and speedometer. that gauge in the cluster is inoperable because the posts on the back are loose from age and probably some questionable removable of the cluster at some point (I was overly careful in my removal and reinstallation because the car has sat for 18 years and I have broken plenty a plastic item on much newer vehicles), so that is currently the only thing aftermarket that is wired in, but utilizing the wiring harness connectors to supply power to the aftermarket gauge without splicing or cutting. that will show power with the key on, but that is the only thing getting power in the interior with a tester.