small rust spot over wiring harness under hood?

[roj2323]

hey all,
it's been a while since i've been on this site but i fially got my 69gmc driveable so i'm now looking at the few rust issues it has.

i have a spot of rust about 1.5 inches across about 6 inches above the wiring harness. it appeers this is actully comming from the inside of the cowl and working it's way out. my question is what causes this and is it a simple fix?

sorry i don't have a photo as this was a last miniute idea to ask.

thanks,
ondrew