75 Duster electrical boxes
You really need a service manual. I have a 76 "somewhere" I'll see if I can find it. Part of the info you need 'may' be in the 73 shop manual which you can download from MyMopar for free
Also, I posted the later wiring section in the manuals thread.......Some of it might be in there
Free service manuals
Here is the wiring we scanned for the 76 wiring
I BELIEVE it is Trailbeast hosted this:
http://www.letsgocomputers.com/docs/76wiringAbody.pdf
Start with about page 9 of that pdf
The box on pass side is EGR time delay