Easy to test the plugs or just replace them. They are a wear item and if you don't know when they were last replaced then do it. If they work at all then not the relay.

Go ahead and replace that fuse link too. It will fail. A lot of amperage goes through it and it heat cycles eventually fatiguing the metal until it fails.

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1986 300 SDL - rolling parts car and test bed.
1987 300 SDL - semi daily driver.
1977 U1000 Unimog
2007 Ram 2500 6.7l