The corner lights and dash lights draw about 90W, and headlights draw 110W according to my own measurements. For a 50% efficient alternator, that's an additional 400W (0.5HP) at the crank.
I've run the numbers through our calculator for a hypothetical Honda Insight in which someone installed a 50% efficient alternator. This car has a larger than typical percentage MPG hit when turning on the headlights, because it spends relatively little energy on aerodynamic drag and rolling resistance.
With a 200W electrical load (ECU, fuel pump, etc.) creating a 400W load at the crank, you'd get about 70.13mpg @ 65mph. Turn on the lights, and you get 67.10mpg @ 65mph. 4% difference.
Lights off
Lights on
The percentage difference in a Ranger is much, much smaller, but the fact that you save about 0.04gal/hour is constant. No matter what you're driving, you can save the same amount of fuel by turning off unneeded 12V loads.
Windows up, fans on is much more efficient in my car than windows down, fans off.