BTW, a leak on the front of the head indicates a timing cover seal leak, not the headgasket...
And the harness would most likely be disturbed if the valve cover was removed. The harness runs alongside the right bank (intake side) of the M104 engine. The plastic wiring cover hides the main harness.
If there is nothing in the service records that indicate a replacement, you can inspect the harness by removing the plastic cover. The harness usually looks pretty new compared to the rest of the ancillary wiring. There is also a white tag attached to the harness with the part number. I believe the newer harnesses have a different part number?
Most harness problems manifested themselves prior to 90K miles of service. MB had tried an environmentally-friendly design which included the "biodegradable" wiring insulation...unfortunately, underhood temps accelerated the process, rendering most harnesses to premature failure.
More than likely, your 131K MB would not have survived this long with the original harness.
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