Rear left indicator faulty, help needed:)

Hi everyone, done a search and can't seem to find anything specific to my fault. The rear passanger side indicator has stopped working, I went down usual route first replacing bulb, then indicator housing replaced, when I went to rewire the unit the wiring sleeve was saturated with dirty water, I've cut about 2ft of the wire out but thecopper wire has rusted and looks black in colour,where is the wire routed so I can replace the entire length? I've traced it to up under rear wheel arch but can't see where it goes from there, or where is the wiring for this located in the engine bay ? Any help/info would bd much appreciated

Thanks in advance

Wiring loom comes out the rear crossmember drivers side, inside the drivers side tub, then runs back out along to the passengers side and up into the tub. Take off the pannels inside the tub by each light unit and have a look for chaffing in there, it could be a bad earth, so run the earth wire from the holder and attach it straight to the tub. If it looks fine it prob will have chafed through inside the chassis. I've cut my loom at each end of the chassis and run new wires ontop of the chassis rail and re connected at each end. Saves messing about!

Edit** simple test, put multimeter on the bulb holder where bulb would sit, if it's reading 12v ish with the indecator switch on but it doesn't light a bulb up run the earth from the holder straight to the tub. This will fix it.. Lol

Cheers maxyman was thinking of attaching externally but wasn't sure where the wiring went, will have a look tomoz