You know you need a new harness when your "friend" messes up the wires that the engine control unit uses to control the fuel injectors. If he doesn't know what he did wrong so that he can undo it, how do you expect a real mechanic to fix it?
I advise you to import one directly from the USA or Europe. I know a dealership in the US with a good parts department that can send you a new harness for US$740 + shipping (less than $100). ELECTRIC CABLE. ENGINE CABLE HARNESS, C/CLK-CLASS. 1995 Mercedes C 220 Sedan # 2025403132
The C180 was never sold in the US, so although the website doesn't list your specific model, they can still order any part in the MB system. Give them your VIN to confirm.
Welcome to the forum.