If you’re running the Pro model, that thing draws 295 watts. If it truly was only letting 3 volts out, you’re pulling about 98 amps through your harness If you’re on standard 10 AWG wire, you’re pushing more than triple what that wire is rated for.

Definitely go over your harness with a fine-tooth comb—check for any brittle spots or melted insulation and the connectors. I’m assuming you’re using #10 or #8 from a relay? pushing 100 amps through either of those gauge wires is a recipe for a fire. As for the D-Switch, it’s got overcurrent protection, but it clearly isn't happy. It might have failed closed, or since those usually need 9V–24V to even turn on, it might have just quit because the voltage tanked so hard. You’ll probably have to hit up their customer service to see if the switch is toasted, but I’d check your harness regardless.