I'd suggest starting at the battery and following the wiring to one of the devices that is dead, checking for voltage at each point in the wiring that you can get to. When you find a spot without power, your problem is between there and the point you checked previously.
Or you can start at the dead device and follow the wiring back to the battery. when you get to a point with power, your problem is between there and previous point.
There is also the possibility that a power relay has gone bad or you lost power to the relay coil.
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