The rear ABS sensor wires go into the car through the wheel arch and then inside the car to the engine bay.Not sure about that wire though,the only other things under the car that would have wires going to them are the lambda sensors.
In Golfs the ABS wire comes out sort of on top of the inner wheel arch,then it joins the main loom and runs along the inner sill.
More common for ABS problems is corroded contacts at the pump,worth having a look there if you can't find anything else.
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