I spent about $80 on wire, tape and heat shrink tubing. I removed the harness in about a 1.5 hours. Spent most of the time removing the stupid plastic covers to expose the harness. The actual removal of the engine harness is not too bad, coils (under intake manifold) and throttle actuator (little clearance to get hands in) are tedious. Once had removed the harness removed all the tape, found the wiring close to the ECM was perfect but any wiring exposed to engine heat, the insulation had deteriorated and crumbled. Spliced new wiring from there to each connector, using the old wire as a length guide. Most connectors was able to directly solder to ends because of the 'awesome' industrial type wiring connectors MB uses which can be un-soldered. I individually heat shrunk each wire at the ECM and then re-wrapped the harness with tape. As recall need about 5 different wire gauges (sizes). Took me about 4-5 hours to do. I am experienced with this type of thing though. I would probably recommend purchasing an assembled harness unless you have experience in soldering and repairing wiring.
I think there are many cars still out there with bad wiring harnesses. But you really don't have too much trouble until you do more extensive engine work like head gasket replacement. Then you move the harness and this movement causes the insulation to come off. This is why so many people have electrical problems after doing engine work on the pre-1997 cars. My car was a late 1996 manufacture, as understand should have had the improved harness (non soy based-bio-degradable) insulation but did not.
Before you get to this remove the plastic cover in the front of the valve cover and cut some of the tape back on the wiring to the EGR solenoid valve/air pump valve and air pump. Check the condition of this wire. If wire ok here the rest of the harness is probably ok. This was the area of worse wire insulation deterioration, on my harness, as exposed to a lot of heat.
This fixed my engine stumbling and dyeing at stops. I also had a bad MAF but found replacing this did not resolve this issue, The MAF wiring to ECM was perfect. Still would stumble at stops. I think what happens is the ECM cannot control, because the wiring going to the DC PM motor inside the actuator is shorted. In my case I found one of the wires feeding the motor had broken (open) because of the deteriorated wire insulation. Even after replacement of wire I had to go thru a ECM reset to get the actuator to operate.
The wiring harnesses are expensive this is why I opted to DIY. I am cheap and since have the skills to fix, I gave is a try and has worked fine for the last several months.