I am wondering if anyone could give me an idea why my horn does not work whenever the C180 (C202-1996-Manual, driven 150,000km) enters a crawling traffic, this happens rainy or dry condition.

The horn works normally during normal driving conditions, but begins to stop working after one minute in a traffic jam. The shop cannot seem to solve the problem, they informed me that they have to open the whole steering wheel mechanism and the air bags.. but before that I hope somebody here can shed some light.