Cracked heads leak combustion gas into the coolant, not oil. The only way for oil and coolant to mix is to have a breached head gasket or some idiot dumping oil in there. If the latter, you should try selling them blinker fluid. They'd prolly buy it.

The box behind the IP is the cruise control servo. The box is the same for most 80s Benzes gas or diesel. The control amplifier is up under the dash above the brake pedal. If you aren't using CC, just leave it disconnected. If you want to fix it, contact forum member "James Dean", he does CC repair and does a good job at it.

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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!)