The wiring diagram in the factory service manual should list the wire size. Let me know when you find the manual, please. I've been looking for a while, no luck.
It's definitely not #8.
You could pull the connector out of the truck, disassemble it, and measure the wire, then you'd know for sure. But unless you plan to replace it with a larger wire, which would be rather tricky to do safely, I really don't see how it makes any difference?