That plug with the three metal do hickeys (snaps)looks like the correct plug for the Aux Pump for the heater. I just fused mine the other day, mine looked same as your picture. Fuse the side that has the Male round prongs. Female is supply and male is load. Fuse the load side. Make sure plug is not all Crumpley and oxidized/weak. Mine was a bit so I wrapped it with electrical tape.

I also squired some black RTV into the space between the 2 male prongs on the load side to keep them seperated. This should help keep the conductors seperated in the future and hold the 2 plug halves shut nicely. The plug halves have snaps and my snaps were a bit worn and not functioning well. This is why I added the black silicone. Permatex Hi Temp in small hand held tube. Great stuff to have around get some. $4 a tube.

I tested my pump with a battery charger on 2 amps and it ran OK before and after I installed the fuse. I just hot wired to male prongs. My pump does not run during Glow Plugging stage but I have good heat. It isn't that cold here so I'm happy that it is atleast fused.

Fusing this pump eliminates the chances of frying the circuit board for the Climate Control Unit. Good luck. Pay special attention to the plugs as they need to be in good condition as they are saddled in a grounded clip spring holder and you are not fusing the supply. I cut some electrical tape in half and make 3-4 rounds with the tape to make sure plugs stay intact. It is hidden with battery in place but it looks way safer now.

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1982 300SD