In the C3 harness find the green wire and check it to see if that is your backup wire
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<LI></LI>[/list]</TH></TR>
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Pin</P></TH>
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Wire Color</P></TH>
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Circuit No.</P></TH>
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Function</P></TH></TR>
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A</P></TD>
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LT GRN </P></TD>
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24</P></TD>
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Backup Lamp Supply Voltage</P></TD></TR></table>