Paul,
My guess is that your column was set up for a stock wheel and when it was changed to an aftermarket hub, the hub is too close to the wheel to allow the dimmer to work, and it also creates problems with the horn. You'll need to loosen the pinch bolt where the column attaches to the input shaft of the steering box and move the column "up" a little (slide towards the back of the car). If it does not move, then you'll need to cut one of the nylon spacers off the column. this is the one that is at the universal joint under the dash at the front edge of the column bearing. Threw will be 2 nylon spacers at this location. One is tapered and has to stay, the other is just a spacer and can be removed.
Also, if you look at the photo of the brass horn isolation plate, I can see a groove worn in it. This is where it was rubbing the turn signal unit itself. This rubbing connects the ground the same as pushing the horn button, even if the horn button is not pushed.
As for the comments above about a wire running down the column, we do not do that. Your turn signal unit (unless it has been changed) is more modern and has an isolation finger for the horn. I do not see any pictures of this on your post. It is likely still attached to the column. Anyway, the only wire you have on your column is a short brown wire that is a jumper across the rubber crush fitting of the column. This simply connects the upper and lower portions of your column, making the upper portion grounded since it is isolated by rubber at all other connections. The other wires on your turn signal unit are all integrated into the unit and the harness of the car.