I don't think the relay is in that location on this car. 77 or 78's, if original, have series glowplugs. If one goes out, they all go out. If original, the plugs get their power from a box on the firewall, juice goes thru all the plugs to the ground at the first plug at the front of the engine. You can tell if they are working because the heavy wires that join the plugs will glow red hot and be visible if the sun is not too bright.
Inside the little plastic box on the firewall from which the plugs get their juices is an 80 amp strip fuse. They can get hairline cracks and fail. You can use a small heavy gauge jumper wire between the screws that hold the fuse to get juice and test the plugs. You could also use a jumper wire from the hot terminal on the battery directly to the first plug at the back of the engine.

If plugs are bad, you really want to convert it from a series loop system to a parallel pencil system. Conversions plugs are available. Search the forum for the part number. The conversion plugs are not the same as the later pencil plugs because the threads are larger. To convert, remove the series plugs, remove the ground wire from plug at front of engine. Use wire from the firewall to energize the first plug at the back of the engine and put jumper wires between the remaining plugs. This will improve cold starts immensely.

Only use ether in an extreme emergency with the glowplugs disconnected.

If you can farm, you can fix it.

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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