I tried my new clamp meter today. If you're prepared to get underneath (keeps you limber... I'm sure I'll say differently 20 years from now!) you can check each individual brake. Just find the two wires going into the backing plate and test each wire. You can check the current going through each wire too, which should detect any shorts at the magnet. If the one wire shows greater current than the other, you have stray current going through the trailer body as it's shorting through the magnet. I'm not sure I trust these clamp meters to detect mA accurately, so I wouldn't pay attention to 100 mA loss or so.
The advantage to testing each side separately is that you could, in theory, have 6 amps going to the brakes. But one side could be shorting, the other side could have resistance at a bad connection. I think this would be a rare instance, though.
Sorry, a longish post to say I've got very close to 3A to each individual brake, just over 6A overall. The brakes probably just need adjusting, and I believe that because I didn't adjust them over the length of our trip from Vancouver BC to LA and back. (They were functioning the entire trip because I could feel them pull the truck back when I tested it with the brake controller).
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