The other method of diagnosis is process of elimination. If you have a junkyard near you with a few of these cars, you can head there and pull these parts. They usually aren't more than $15/ea. Then just start swapping to see if things change.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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