If this is on the left side (which it appears to be) check your battery tray for acid leakage. This appears to be a weak point in the design of the W-126's. You'll probably find the trail of corrosion leading down the firewall.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now