Before replacing the harness, are you sure it isn't just a bad oil level sensor? They are known to fail, the one in my SDL is that way, it randomly comes on and off despite a full pan. The wiring harness is fine (and not made of the dissolving insulation), but the sensor is all mucky. I've just been too lazy to change it. That light is easy to ignore.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)