Sadly, there's unlikely to be a quick fix for this one, if I were in your position I'd go over the individual circuits so I could be happy there wasn't an electrical fire hiding away. Joe has put a link to the wiring diagrams so if you don't have one yet, that's the starter;
http://www.lotuseuropa.org/LotusForum/index.php?topic=752.0I usually print & laminate a copy, easy to wipe clean and more robust when you're working on the car.
The horn might be a simple fix. It works by having a live feed (green wire) from the fuse box to the horn with the earth return circuit (purple/black wire on my car) going up to the horn push via a brass slip ring that's situated at the top of the column, just below the steering wheel itself. When you press the horn push it goes to earth via the steering shaft/column and sounds the horn.
If yours is going off without a horn push simply be heaving on the wheel itself, then you're earthing the circuit. Look for damage or trapping on the steering column wire (purple/black) and then the brass slip ring; that should be insulated from the outer steel of the column by a plastic carrier and be well away from the inner steering shaft, which is also an earthed item.
The other problems sound like someone's just messed up connections and could be anywhere, hence the need to check individual circuits. I think the window switches and rad fan are both on the green wire fuse with the windows permanently live from the fuse feed side (effectively un-fused) but I've modified my car so check on your diagram.
Final suggestion is that unless the wiring is recent then clean and re-make any earth connections as you find them, especially the light units or anything exposed to weather. Poor earths are regular bugs with old fibreglass cars.
Brian