The wiring harness is all of the wires in the engine bay. The early 104 engines used poor materials where the wire covers would all flake off from the heat. This causes all sorts of random problems within the engine!! If you do a search, you'll come up with all sorts of information on this topic. IF you have 100k miles or less on your car, sometimes MB will replace it for free or split the cost. Are you the original owner of the car?
Cost is about a grand to have it done. Be sure and do a search, you should find a fair amt of info on the topic.
Inspect all wires in the engine bay for flaking covers! Good luck (or actually not, hope it's an easier fix than that...!)
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)