When I looked at the 451 wheel back in 2007 I could see a weak point in the system.
I'm sure it'll soon become a common problem. Smart knew it'd be a problem as they changed the design in 2010.
Disassemble the steering wheel and 1st look at the back of the airbag. There is a small spade connection.
This is the less likely cause of the issue. So far, this part hasn't been the cause but it could be in the future.
Below is the part that causes all of the horn faults so far.