Just look at wire colours going to relay holders, to see if some the same as switch.
The fuse no doubt is the main feed for the HFS, so you are only missing the switch supply(power).
My cossie did this so couldn't be arsed with in depth ripping to bits, just jumped it from the heated REAR screen supply, works perfect, I know that is not how it should be but short of tracing the supply wire all the way back to the break which could be anywhere there are not many choices.
tabetha