Why do I hear water splashing sounds and what can cause electrical problems?

Hello, it sounds like you have multiple concerns that are likely not related to each other, unfortunately. Water sloshing could be evaporator moisture collecting due to a drain tube routing or clogging issue. It could also be air pockets in the cooling system for some unknown reason. The lack of heat can be a temperature blend door issue concern, or an engine cooling system concern. The horn issue can be related to the horn pad, the air bag sliding contact / clock spring or the wiring. The windows and lights can be blown fuses -- check this first, faulty switches, window motors, and so on. Testing is needed. I'd probably suggest finding a tech local to you who can be of reliable and reasonable assistance!