Don't feel bad. After swapping out the OEM shock absorbers for a set of adjustable Ranchos, I started hearing a rattle. Paid the dealer over $150 to find I had failed to sufficiently torque the left-front lower mount. If the plug was in your harness, then I assume you're not the original owner, and the first owner had at some time removed the tailgate. That plug should've been in a sealed bag in your glovebox when purchased new.