"They were flexible, no insulation showing so I'm guessing that it's been replaced at some point"
It's the other way around. The insulation should be showing. In other words, the wires should like as you'd expect them to. You should see white, green, red, black, etc. If you saw the insulation flaking off and you saw the bare copper wires showing underneath, then you're in trouble.
In any event, when you're done, wrap up the area that you cut open with black electrical tape.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".