OK I will forget about helping with the troubleshooting part and just answer the harness part.
There are wiring harnesses. I use universal, pre wired fuse panel type. Some crimping involved. About $200.
Example:
Click
A near "plug and play" type is about $600 for 63-66 Chevy. There are a few differences on GMC not covered. But that's OK.
Building your own is not viable.
This post should be in the electrical forum.