The harness on the M104 runs along the firewall just above the rear of the valve cover and then parallel to the driver's side of the cover.
Unfortunately, the insulation damage is internal to the harness. In other words, even a defective harness will appear to be sound from the outside...that is, until you skin off the outer wrap to reveal the deteriorated wiring insulation inside. There is a thread with a pretty vivid picture of one.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car