I started looking at the fuses today. Strangely my VW book that came with the car has a slightly different fuse panel layout than I actually have.
Horn: The fuse was blown but when I replaced it, no dice. Doing a bit of googling (actually Bing'ing) I came across a few thread discussing relays. Apparently the horn is a #53 relay in spot 11. I don't really know what this means. I don't actually know what a relay does here, though I take it it's related to the sort switch.
Seat heat: I disconnected the passenger side heater. Using a multimeter, I tested for power. Now I'm no electrician, but I was surprised when the meter read 0.93. Is that, volts or amps? Switching the red/black read -0.9 or so, so I'm assuming it's volts. When adjusting the switch to 0, the meter read 0.0 as well. So the switch does something t least.
Mirrors: No progress. I'll take a look at the wiring harness another time.