It is easy to trace the high resistance that is causing this just very time consuming.
There is obviously a point where the power is fed after the igntion switch to various components, including one point that feeds these two things, that point has to be between the ignition switch and the fusebox, it could be a corroded bus bar for example, or a soldered joint, or course a simpler way around it some have done is to go to the underside of the fusebox, and reroute another live fed from the ignition switch to the input side of the fuse holder(blower fan), to restore it to full 13/14 volts, that will of course restore any ohter circuit that is joined to it ie the horn also.
I don't do it this way but can see why some do, other wise it is a case of following the harness to see where joins are.
tabetha