I agree with what everybody above has said. I would add that you should get a multimeter, if you don't already have one. Don't make any assumptions as you start to run the diagnosis. It could be a bad connection at the Firewall plugs, a bad switch, bad ground, bad bulb/horn, etc. You need to check everything. When experts like
@67Dart273 weigh in, I feel kind of silly adding my two cents worth, but I'm fairly good tracking things down.
Diagnose one thing at a time. For example, start with the horn. Have an assistant press the horn button while you check for power at the horn. If there is power, but the horn does not work, it is either a faulty horn or a bad ground. If there is no power, then the problem is further upstream. Nest I would check the horn relay. Power goes from the relay out to the horns when the horn is pressed. When the horn button is pressed, power will flow to the relay, and if the relay is working, power will flow out the other side of it to the horns. If there is power going into the relay but not out the other side, the relay is bad. If there is no power going to the relay when the horn button is pushed, the problem is further upstream. Time to pull the steering wheel. The power wire going into the horn switch should be hot all the time. The horn switch works by completing a ground and allowing the current to flow, clean the copper ring on the back of the steering wheel and check all connections.
The same strategy can be used for all other items. It is pretty much a bad switch, bad item (horn, bulb, fan motor, etc), bad relay, bad ground or a bad connection/broken wire somewhere. I always start at the item that isn't working and work my way backwards. In fact, it could even be a fuse.