Replacement of upper and lower wiring harness' is very straightforward....but check the condition of your existing harness before replacement.
Remove the airbox and check the harness that is visible - by the injectors, throttle body etc...check for cracks and splits in the wiring...the harness gets brittle from age and engine heat, so no big surprise.
The harness' are very approx 300gbp each.
There are some fol who think these cars run hotter than other V8's...in my experience, certainly not the case. With a correctly funcitonal rad, thermostat and good catalytic converter, these cars will run at a steady 90degC, even on the hottest summer day with AC running, it will barely get above 100. Both my 500E's run this way.
cheers
talbir