Switch? You mean the actual switch that locks and unlocks the door? that doesn't make sense. It would mean that both switches on the left and the one on the right all went. And in that case it shouldn't do ANYTHING; same for a fuse.
You are thinking of the switches as if they are in parallel when in fact they are in series. Here is the schematic below. After looking at the schematic it points towards the passenger switch. If you wanted to test it, you could jumper a known 12v hot to the actuator lead. I would do this first as the actuators are a *****, I think they are riveted inside the door.
P.S. My car is manual; and I am already feeling the difference in gas mileage vs my old toyota corolla. But the mustang is just sooooooooo much more fun to drive.
Some quick math: 3 MPG difference; let's say 10 gallons to fill up the v6(don't know correct amount, it's a little more). That is 30 miles lost per fill up in the v8. Gas is 3.07 here a gallon for low grade, and v8 gets 15 mpg. That means I would lose approximately $6.14 a fill up. After already paying $30, basically now it becomes $36 to fill her up? No thank you.
That's fine that's your preference. To some people the .87 cents extra a day ( your example, 1 tank a wk) would be worth it to have the extra power. Just saying it's not a huge difference that you claimed it was.