where does this connection go?

Hi everyone! I bought a used classic about two months ago, did a complete teardown to clean the case, descale boiler, replace tubing etc. I didn't have time to finish the project until today, so I finally decided to put it all back together and ended up with a wire which I can't place - can anyone help me and tell me where it goes?

I took pictures and numbered everything, but either it was hidden behind other stuff in the photos, or I just messed up and skipped one.