[AZKen]

I guess you want to wire in the new headlight harness. Is that right? Are there any instructions? If so, can you post? If not, can you post pics of the actual harness you have, showing each wire and connector. Spread out, not a stock web pic.

I am a bit confused still. Did you buy two harnesses? If so, why? You do not need heavy duty. You do not need relays. Slow down. You are getting carried away and spending money for items you don't need. You can buy a whole harness for $200. For the whole truck. So what did you already buy? What headlights are you installing? LED, Halogen, sealed beam...... Do you have an alternator or a generator? Are either of those new?

I agree with Lizzie 100% except for the relays. So I guess 99%. He is knowledgeable. We can differ by 1%.

The diagram you found is full of errors. Someone took a Chevy diagram and tried to turn it into a GMC diagram by looking at the official GMC diagram and using a cad program. Also, I do not think the harnesses you bought are actually for Chevy or GMC. I think they are Chevy. These two things are what is going to confuse you. If you wish, I will help you thru this mess if you answer my questions above.