It's a very simple circuit, but you need to understand what it is doing. The relay is in place to convert a switched ground to a switched 12v. This is done by the switch (horn button) providing ground to the relay which should always have 12v on the input side.

The column must be grounded. Some cars may need the ground wire while others, due to worn paint, etc. can provide the ground through the mounting points. Remove the horn button and check for continuity to the relay. Also check the ground on the column.

You also need 12v input to the relay. That is easy to check with a meter, too.