Ground otter switch

[Mich7920]

I just had a ground problem on the header tank. The new otter switch did not start the fan. After some investigation, I understood that it was'nt at all connected to the ground.
The front frame has just been repainted. the bush of the header tank and the radiator are new.
I looked at all the photos taken before the restoration without discovering any additional ground for otter switch.



Of course after added a ground frame/braket it works perfectly.
Was it necessary to remove paint near the bush or did I forgot something ?