The wiring diagrams for the radio itself is in PDF files for the owners manuals available on RF's website .
I have them saved to my PC too .
As for using an adapter to the factory wiring , you will not have the memory 12 volt wire in a 76 blazer at the wiring plug . You'd have to tap into either the lighter circuit , or into a bat terminal on the fusebox ( preferred ) . I used the lighter circuit in one of my trucks , but have no lighter anyway .
You original truck wiring is :
yellow = 12v+
black = ground
gray = light dimmer from headlight switch
I don't know the speaker color code , as the speakers harness plugged right into the radio , and I don't have the factory speaker wiring anymore . My other truck had just the mono speaker in the dash and that one had the speaker right to the radio too .