The sub-woofer isn't connected to the wiring harness of the KDC-400U. It has to be wired separately. Then the sub-woofer input gets connected to the KDC-400U sub-woofer out using R/L audio wires as show on page 13 of the Quick Start Guide. The direct link to the Quick Start Guide is
http://manual.kenwood.com/files/4fd9a24acb9fa.pdf . You may be able to use the wiring information on pages 12-13 to help you determine which wires aren't going to the sub-woofer.
Most Kenwood Car audio manuals are available here:
http://manual.kenwood.com/en_contents/search/ .
I wish that I could give you more information.
Cindy Wells
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