Easy test -- take a standard 9V battery and attach it to the leads. Note which orientation causes the cone to travel outward, and then see which wire is touching the + battery post. That's your + lead.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)