Very likely it's the original then. As I said, carefully split an inch or two of the outer sheath open with an X-acto knife and check the condition of the wiring inside. Do this near the engine, not near the firewall. You'll likely find your issue after you see the condition of the wiring inside.

Rule of thumb is that if you don't have documentation that a harness has been changed, 95% probability that it has never been changed. Be prepared to pay around $700 for a new factory harness (no aftermarket units made) from MB. Cheapest place to order is through XXXX.

Cheers,
Gerry