I agree completely with Jim H and Eskimo. If you're uncertain about which is hot and which is neutral, get the small two pronged tester. If you hold one prong in your palm, and touch the other to a hot wire, it will glow faintly (no shock from that, it's recommended usage); touch one to the hot and one to neutral it will burn brightly.

The light will still work but it's worthwhile to do it right.

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1986 300SDL, 362K
1984 300D, 138K