i dont know what year your car is so i used my 96 sc300 to get the diagram. the ignition switch diagram is below.
since you said u have narrowed it down to NOT being the ign switch i take it you ohm checked it and i wont go into detail there. since there is only 1 wire in and 1 wire out have you followed the from the 100amp fuse to the 40amp there is a white/blue wire, and from the 40amp fuse to the ign switch there is a white wire. start by inspecting those 2 wires to see if you can find a chaff or damaged wire and repair it and the source of the damage. if you can not find any damage on the wire reply and we will go from there