Got some rear builds replaced at a service yesterday. Now the reversing beep happens when I indicate left and brake. Skoda tell me the systems are not connected at the light cluster so not worth me poking about in there.

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I'd check the earth connections, but I'm old school.

"Builds"? Do you mean bulbs? I'd take a close look at the light cluster - probably a wrong bulb or broken filament shorting - all sorts of weird things can happen or perhaps a poor earth as BRYANB says