The reason for this little harness: The early rigs only had a single ignition on switch contact, they only ran a relatively small number of loads, ignition coil, carb idle cut, etc. The later rigs have a double switch to deal with higher loads, like injectors, fuel pump, etc. So this relay circuit is to take the load off of the switch. There are actually two relays, mounted side by side, the other one runs the ign, EFI, etc, this one is for key on accessories, heated seats, ARB control, etc.
Anyway, the relay is a factory headlight unit, pretty stout, ~40A. The plug is cut from the relay panel. The W/BK (ground) wire connects to the wire left hanging out of the relay next to it, the BK/BU connects the ign on from the key switch, these close the relay switch. The R/Y connects to the main battery input, supplies power, the Red wire sends power, split to three fuses. So when the key is off, the fuses are dead, when the key is on, they have power, with very little draw from the key switch.