This is a 315 Page PDF covering the Electrical Layouts of the GM C/K (GMT400(?)) Full Size Pick Ups. Not certain if it is complete... but it looks quite comprehensive and in any case... it might have some value as a research document:
https://www.gmupfitter.com/publicat/2004_BB/2003_Beyond_LD_Electric_CK.pdf
https://www.gmupfitter.com/publicat/2004_BB/2003_Beyond_LD_Electric_CK.pdf