My recommendation would be to start by disconnecting all 4 of the wires that are fed by that circuit, then measure each one to ground. If you're lucky, only 1 of those wires will have a low resistance path to ground. Once you identify that one, trace that wire to determine what it powers. Or leave it disconnected, reconnect the other three wires, then see what doesn't work and that's probably the circuit of issue. Maybe during some reassembly a screw was run through a power wire, or when something like the seat was mounted it pinched and shorted a wire, etc.
I thought Painless wiring was the type that had writing on the wires for what circuit they went to. Although if this wire to the battery did not have fuse protection prior to you adding the fuse, it's probably not something that came from Painless to begin with....
-Rod