Is there any service manual out there that shows the routing of a 1st Gen Blazer's wiring harnesses and locations of connectors?

I've got the X-8529 GMC S-Truck service manual. It has a wiring schematic, but it would help me if there was one that showed where the various harnesses run and connect.

 

Try rockauto.com and look at the literature for your truck. Assuming you are looking for those of an 85 S10 Blazer, the OEM service manuals are available also on CD-ROM. Not 100% sure but these manuals should also have the electrical wiring diagrams. The correct manuals are NOT the Chilton or Haynes.