75 and 76 has no easy answers, or maybe "cheap." You are going to have to search around and either find an origininal, or "repop" / reprinted, or "on CD" service manual for that year. The wiring over the years has changed some, and you need the correct manual.
MyMopar.com is the website the guy above was thinking of, but they do NOT have the correct manuals
They do have aftermarket, 2 page manuals you can download --in the wiring section-- but those are NOT detailed enough, they are, however, somewhat easier to follow for some problems in order to understand the system. In other words "I use both."
However, are you intending to buy used/ new harnesses, that is reproduction harnesses, or are you going to use a more "universal" approach, which many do?
If you are going to use a universal harness, that of course is even more intimidating, as they do not follow original wiring, and that is sometimes a good thing. The universal harnesses are better fused with more circuits.