Ground is typically brown or black wires. You could always just run a new wire and ground it to the chassis to be safe. If it doesn't work, switch the power wire to the other one.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth