is it on the passenger side fore wall with 4 fuses in it?

I'm not familiar with the 115 but I seriously doubt that's it. Follow the wires from the glowplugs back to the relay. If it's anything like the early 123's the wire will first go to a fuse box with a heavy strip fuse and then into the car.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

its behind the dash on the drivers side.

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Aye. Same as my early, early W123 300D. It's hanging from the bottom of the steering column, I think. Big. Shiny. Aluminum. Can't miss it. I pulled one from a '75 300D, and it's the same part. It's been a while.

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- K.C.Adams

'77 300D Euro Delivery
OM617 turbo / 4-speed swap
404 Milanbraun Metallic / 134 Dattel MB-Tex

Current status:
* Undergoing body work

My '77 300D progress thread