That's only a portion of the wiring on a 26 year old Heil, on it's side in an attic but still as big as a damn basement freezer. There are no less than five relays on it, a couple being added later for whatever purpose. Apparently on this model they had separate xfmers, relays and everything for both heating and cooling. I couldn't wrap my head around it in the brief time I was up there.
Another contractor had so mangled the job of putting in a heat pump over this past vicious summer that the H/O gave up and got by on a window unit. Now the blower will not come on after the furnace fires (thank God it did fire on emergency heat).
I took one quick look at that mess and grabbed L1 off the primary side of the heating xfmr and put it to a low motor speed, verified common and let 'er rip long enough to make sure it was enough air to avoid tripping on limit. Continuos blower operation while d/c is on and heat will cycle via stat to get him by until we can replace everything.
Ironically, I had just received my RSES Residential Troubleshooting book which seems to have a very detailed section on reading schematics.