The ELR plug is on the back of the injection pump. It's a red solenoid with a plug on top. If you unplug it and the engine speed suddenly drops, you know the ELR is doing something.

Not sure if the 350 has the ELR adjustment knob or not. If you do, it's on the firewall behind the fusebox. It's numbered 1-7

__________________
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!)