Answer: They are related in the sense that the person who installed the steering wheel apparently didn’t give much forethought to the effects of the installation, or do a proper job with the installation itself.
It is possible that the horns themselves are defective, but it is more likely that the installer did not install a “horn kit,” or installed it improperly. This should be relatively easy for a mechanic to rectify.
The issue that the car is hard to turn at slow speeds is due to the fact that the aftermarket steering wheel has a smaller diameter than the original. There is a reason that these older cars had huge steering wheels. They provide leverage when turning the vehicle at slow speeds. I would be willing to bet that your car does not have power steering. If it did, you would not be experiencing the problem. Your solution is to add power steering, or replace the steering wheel with a larger diameter unit such as the original one.