When I step on the brakes in my 2010 Toyota 4Runner, I do not get any DC voltage to the "electric brakes" pin of the 7-pin trailer socket (factory installed). (I measure this by touching the probes on my multimeter to the "electric brakes" pin and the "neutral/-" pin at the same time while the brake pedal is fully depressed.)
Every other trailer socket pin produces voltage as expected (i.e. 12V/+, running lights, left light, right light; admittedly I did not test reverse lights). Note that the brake lights do work, but of course that's just the left and right lights powered at the same time.
I pulled and checked every fuse under the hood and in the driver foot well, triple-checking any associated with "towing" or "trailer". I also confirmed that both relays associated with "towing" or "trailer" are functioning correctly; not only do they click, but they have minimal resistance (0.2 Ohms) when powered.
Note that I do not have any trailer brake controller installed. (I know there's a factory-installed connector for it in the driver foot well; there's nothing connected there.)
I've heard there might be a towing relay behind the panel on the driver's side of the trunk. Should I pull that panel next?
What's the next troubleshooting step I should perform?