i pulled this from a friend's 240D that was sent to the crusher. the motor was always really beat but i think having manual control over the plugs was the only thing that allowed it to start. i usually see these things with wires between the plugs that replace the original wavy metal pieces but the guy that owned the car before them was a real wingnut and i guess he figured out how to make it work with the original equipment. i think you need the large pencil plugs to make it work but i'm not 100%. that's what was on the car though. i've attached a wiring diagram that i drew up before pulling this from the car. it hooks into the existing glow plug harness at the firewall. but i dont believe it still needs the original gp relay, because i didn't see it when pulling stuff from the car. pushing the button does light the GP bulb on the cluster though. i dont know if it just intrinsically works like that or if there was modified wiring at the cluster. this was on a 240D but i'm guessing it could also work on a 300d. I'm also guessing it could be modified to work on 81-85 cars but i dont know? the blue wire is run through the firewall and the button was mounted under the dash. i believe the large bolt at the end was a chassis ground.

the wires are really heavy and stiff. that enormous relay was mounted to the valve cover but if you extended the wire i'm guessing you could mount it almost anywhere. that two-post thing was mounted to a bracket that was mounted to the battery tray arm (with two holes drilled out by the guy).

make an offer? i want to see it go to a good home

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1981 NA 300D 310k miles