The GP relay in my '83 is behind the driver side headlight just in front of the wheel well. Pretty sure it's the same in a '78 but I don't remember 100%. I'd check there first.

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Ich muss geisteskrank sein.

The 1983 300 SD: "Old Smokey" (Hilda) Quarter Million Mile Club

The 1986 420 SEL EURO: "¡El Conqístador!" (Helmut) Trunk Triangle Club

The 1995 Cadillac Coupe deVille: "Schlag Kaltluft" (Shaft)