A wiring harness is just a tidy set of plugs with the wires already hooked up. No issues at all building your own as long as all the components are rated 40@ or higher. Or at least as high as the fuse you are going to put in. The fuse always needs to be rated equal or preferably less than the weakest component(s) in the circuit.I have a 40amp relay with wire harness plug but the wires on the harness are not 10ga. Can I just use the blade type connectors that are made for 10ga wire and hook them directly to the relay instead of using the wire harnes plug that came with it