I had the same problem last winter and was also unsure if it was the plugs or the relay. Could not start it at 12 degrees. A weak battery added to my woes. But, I made a little jumper with five leads on one side and all the wires twisted and soldered toether on the other end. Stick each of the five wires into the sockets in the plug (after disconnecting it from the relay) and run a heavy jumper cable directly from the battery to the single soldered lead. Let is sit about 30 seconds while still connected and attempt to start the vehicle. In my case, two plugs were bad and this solution did not work. The relay was fine. But, I keep the jumper in the trunk just in case I need it in the future

Brian Carlton
1984 300SD