Need help with grounds, stay positive cable

[Guy Dudebro]

In restoring my 67 c10, I'm having trouble mastering the electrical system. I have a slow drain on my battery somewhere and I think it may be an insufficient ground?
I had my battery charged and it was at 13.1. Next time I check, 12.7. Then 12.4 and so on.
I think maybe it was because I had the 4 gauge wire on the negative cable on the plastic of the fan shroud.

I attached pic of the positive and negative.

Questions:
Where should I ground the negative cable?

I took out the AC. So what should I do with the stray cable that DOESN"T go to the starter on the positive cable?

Any help is appreciated.