I believe that rebuilding the harness will take some time but you can then use any quality wire you want, industrial, military etc.

I remade a starter harness, very expensive for some few lousy wires when the connectors were perfectly fine. I got the original "tesa" brand tape from ebay too, once completed like it should tape it up nicely and you have a factory looking harness for cheap.

I have experience in building engine wiring harnesses by hand - which were usually required when a JDM engine goes into a USDM car etc etc. The RHD harness is completely useless in some cases. These old MB ones are not that terrible - Infact I like it that MB placed the ECM in the engine bay and not buried under the dash to give you back spasms while working.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)