Thanks to hatzie, you can download the year-specific wiring diagrams here -->
http://67-72chevytrucks.com/vboard/s...d.php?t=545416
As a general rule, the factory wiring used the same color wire throughout each circuit. So the splice from red to yellow (or tan) wires is an indication that someone has made some modifications with wire colors that likely won't match the diagrams. That means you might have to do some additional detective work, tracing the wires to see where they go.