The stop light switch is a City-El Mystery of long standing.
It is again and again defective.
This issue has long history. I've tried simple micro switches of different contact loads up to micro switches provided with blue rubber caps.
Very few City-El have not had problems with this switch.
Why and why the switch is again and again defective, remains hidden for me. The load by two lamps 21W should actually be no problem.
There were alternative solutions already with relays or transistors downstream.
Some contend that vapor from the closely situated brake fluid container forms an insulating layer on the switch contacts - whether this is correct I know not.
The only alternative sure to function is installing a standard KFZ (NAPA or Standard for us Yanks-RRY) stop light switch on an auxiliary angle.
(For US readers, this is the box my replacement switch came in, and this is the instruction sheet that came with it. I have several months of satisfactory service on this switch. I made up a little pigtail of wire with crimp-on slip connectors as the existing wire would not stretch to reach the new switch. I'll post pictures of my installation soon. - RRY)
Who wants to change nevertheless the normal stop light switch, the brake pedal must remove.
- work on brake assemblies is something for specialists -