How to tell, absent documentation, if M104 wiring replaced?
 

So . . . I'm out looking at a M104 300E or 300CE to purchase. If the seller does not have documented evidence, how does a semi-literate, uninformed purchaser tell if the engine wiring harness has been replaced? Exactly where, under the hood, do I look to see either cracked, original wiring, or if it has already been replaced? Thank you.

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.

There are many places to check the wiring harness to see if the insulation is flaking off.

One place is right at the the terminations on the thermostat housing you'll see the insulation at the ends coming off.

Another place is where the wires in the plug to the intake cam advance magnet are. It may be necessary to remove the plastic connector from the wires to see the insulation.

You can also take a utility knife and cut the black fabric covering the harness that runs along the fire wall.

at one end of the harness is a big square plug. somewhere near that plug is a white tag which has part number and year of manufacture information. if the date is recent then you have a new harness. if the date of manufacture is '93 - '95 it is probably original or, if it was replaced, is suspect again.